thp: update documentation
The patch updates Documentation/vm/transhuge.txt to reflect changes in THP design. Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Acked-by: Jerome Marchand <jmarchan@redhat.com> Cc: Sasha Levin <sasha.levin@oracle.com> Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Hugh Dickins <hughd@google.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Mel Gorman <mgorman@suse.de> Cc: Rik van Riel <riel@redhat.com> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> Cc: Steve Capper <steve.capper@linaro.org> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Michal Hocko <mhocko@suse.cz> Cc: Christoph Lameter <cl@linux.com> Cc: David Rientjes <rientjes@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -35,10 +35,10 @@ miss is going to run faster.
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== Design ==
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== Design ==
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- "graceful fallback": mm components which don't have transparent
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- "graceful fallback": mm components which don't have transparent hugepage
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hugepage knowledge fall back to breaking a transparent hugepage and
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knowledge fall back to breaking huge pmd mapping into table of ptes and,
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working on the regular pages and their respective regular pmd/pte
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if necessary, split a transparent hugepage. Therefore these components
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mappings
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can continue working on the regular pages or regular pte mappings.
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- if a hugepage allocation fails because of memory fragmentation,
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- if a hugepage allocation fails because of memory fragmentation,
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regular pages should be gracefully allocated instead and mixed in
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regular pages should be gracefully allocated instead and mixed in
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@ -221,9 +221,18 @@ thp_collapse_alloc_failed is incremented if khugepaged found a range
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of pages that should be collapsed into one huge page but failed
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of pages that should be collapsed into one huge page but failed
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the allocation.
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the allocation.
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thp_split is incremented every time a huge page is split into base
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thp_split_page is incremented every time a huge page is split into base
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pages. This can happen for a variety of reasons but a common
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pages. This can happen for a variety of reasons but a common
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reason is that a huge page is old and is being reclaimed.
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reason is that a huge page is old and is being reclaimed.
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This action implies splitting all PMD the page mapped with.
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thp_split_page_failed is is incremented if kernel fails to split huge
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page. This can happen if the page was pinned by somebody.
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thp_split_pmd is incremented every time a PMD split into table of PTEs.
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This can happen, for instance, when application calls mprotect() or
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munmap() on part of huge page. It doesn't split huge page, only
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page table entry.
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thp_zero_page_alloc is incremented every time a huge zero page is
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thp_zero_page_alloc is incremented every time a huge zero page is
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successfully allocated. It includes allocations which where
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successfully allocated. It includes allocations which where
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@ -274,10 +283,8 @@ is complete, so they won't ever notice the fact the page is huge. But
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if any driver is going to mangle over the page structure of the tail
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if any driver is going to mangle over the page structure of the tail
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page (like for checking page->mapping or other bits that are relevant
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page (like for checking page->mapping or other bits that are relevant
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for the head page and not the tail page), it should be updated to jump
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for the head page and not the tail page), it should be updated to jump
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to check head page instead (while serializing properly against
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to check head page instead. Taking reference on any head/tail page would
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split_huge_page() to avoid the head and tail pages to disappear from
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prevent page from being split by anyone.
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under it, see the futex code to see an example of that, hugetlbfs also
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needed special handling in futex code for similar reasons).
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NOTE: these aren't new constraints to the GUP API, and they match the
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NOTE: these aren't new constraints to the GUP API, and they match the
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same constrains that applies to hugetlbfs too, so any driver capable
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same constrains that applies to hugetlbfs too, so any driver capable
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@ -312,9 +319,9 @@ unaffected. libhugetlbfs will also work fine as usual.
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== Graceful fallback ==
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== Graceful fallback ==
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Code walking pagetables but unware about huge pmds can simply call
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Code walking pagetables but unware about huge pmds can simply call
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split_huge_page_pmd(vma, addr, pmd) where the pmd is the one returned by
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split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by
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pmd_offset. It's trivial to make the code transparent hugepage aware
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pmd_offset. It's trivial to make the code transparent hugepage aware
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by just grepping for "pmd_offset" and adding split_huge_page_pmd where
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by just grepping for "pmd_offset" and adding split_huge_pmd where
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missing after pmd_offset returns the pmd. Thanks to the graceful
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missing after pmd_offset returns the pmd. Thanks to the graceful
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fallback design, with a one liner change, you can avoid to write
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fallback design, with a one liner change, you can avoid to write
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hundred if not thousand of lines of complex code to make your code
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hundred if not thousand of lines of complex code to make your code
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@ -323,7 +330,8 @@ hugepage aware.
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If you're not walking pagetables but you run into a physical hugepage
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If you're not walking pagetables but you run into a physical hugepage
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but you can't handle it natively in your code, you can split it by
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but you can't handle it natively in your code, you can split it by
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calling split_huge_page(page). This is what the Linux VM does before
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calling split_huge_page(page). This is what the Linux VM does before
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it tries to swapout the hugepage for example.
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it tries to swapout the hugepage for example. split_huge_page() can fail
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if the page is pinned and you must handle this correctly.
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Example to make mremap.c transparent hugepage aware with a one liner
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Example to make mremap.c transparent hugepage aware with a one liner
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change:
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change:
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@ -335,14 +343,14 @@ diff --git a/mm/mremap.c b/mm/mremap.c
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return NULL;
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return NULL;
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pmd = pmd_offset(pud, addr);
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pmd = pmd_offset(pud, addr);
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+ split_huge_page_pmd(vma, addr, pmd);
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+ split_huge_pmd(vma, pmd, addr);
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if (pmd_none_or_clear_bad(pmd))
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if (pmd_none_or_clear_bad(pmd))
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return NULL;
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return NULL;
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== Locking in hugepage aware code ==
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== Locking in hugepage aware code ==
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We want as much code as possible hugepage aware, as calling
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We want as much code as possible hugepage aware, as calling
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split_huge_page() or split_huge_page_pmd() has a cost.
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split_huge_page() or split_huge_pmd() has a cost.
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To make pagetable walks huge pmd aware, all you need to do is to call
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To make pagetable walks huge pmd aware, all you need to do is to call
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pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the
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pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the
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@ -351,47 +359,80 @@ created from under you by khugepaged (khugepaged collapse_huge_page
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takes the mmap_sem in write mode in addition to the anon_vma lock). If
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takes the mmap_sem in write mode in addition to the anon_vma lock). If
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pmd_trans_huge returns false, you just fallback in the old code
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pmd_trans_huge returns false, you just fallback in the old code
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paths. If instead pmd_trans_huge returns true, you have to take the
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paths. If instead pmd_trans_huge returns true, you have to take the
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mm->page_table_lock and re-run pmd_trans_huge. Taking the
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page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the
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page_table_lock will prevent the huge pmd to be converted into a
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page table lock will prevent the huge pmd to be converted into a
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regular pmd from under you (split_huge_page can run in parallel to the
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regular pmd from under you (split_huge_pmd can run in parallel to the
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pagetable walk). If the second pmd_trans_huge returns false, you
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pagetable walk). If the second pmd_trans_huge returns false, you
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should just drop the page_table_lock and fallback to the old code as
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should just drop the page table lock and fallback to the old code as
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before. Otherwise you should run pmd_trans_splitting on the pmd. In
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before. Otherwise you can proceed to process the huge pmd and the
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case pmd_trans_splitting returns true, it means split_huge_page is
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hugepage natively. Once finished you can drop the page table lock.
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already in the middle of splitting the page. So if pmd_trans_splitting
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returns true it's enough to drop the page_table_lock and call
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wait_split_huge_page and then fallback the old code paths. You are
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guaranteed by the time wait_split_huge_page returns, the pmd isn't
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huge anymore. If pmd_trans_splitting returns false, you can proceed to
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process the huge pmd and the hugepage natively. Once finished you can
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drop the page_table_lock.
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== compound_lock, get_user_pages and put_page ==
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== Refcounts and transparent huge pages ==
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Refcounting on THP is mostly consistent with refcounting on other compound
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pages:
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- get_page()/put_page() and GUP operate in head page's ->_count.
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- ->_count in tail pages is always zero: get_page_unless_zero() never
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succeed on tail pages.
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- map/unmap of the pages with PTE entry increment/decrement ->_mapcount
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on relevant sub-page of the compound page.
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- map/unmap of the whole compound page accounted in compound_mapcount
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(stored in first tail page).
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PageDoubleMap() indicates that ->_mapcount in all subpages is offset up by one.
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This additional reference is required to get race-free detection of unmap of
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subpages when we have them mapped with both PMDs and PTEs.
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This is optimization required to lower overhead of per-subpage mapcount
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tracking. The alternative is alter ->_mapcount in all subpages on each
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map/unmap of the whole compound page.
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We set PG_double_map when a PMD of the page got split for the first time,
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but still have PMD mapping. The addtional references go away with last
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compound_mapcount.
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split_huge_page internally has to distribute the refcounts in the head
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split_huge_page internally has to distribute the refcounts in the head
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page to the tail pages before clearing all PG_head/tail bits from the
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page to the tail pages before clearing all PG_head/tail bits from the page
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page structures. It can do that easily for refcounts taken by huge pmd
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structures. It can be done easily for refcounts taken by page table
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mappings. But the GUI API as created by hugetlbfs (that returns head
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entries. But we don't have enough information on how to distribute any
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and tail pages if running get_user_pages on an address backed by any
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additional pins (i.e. from get_user_pages). split_huge_page() fails any
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hugepage), requires the refcount to be accounted on the tail pages and
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requests to split pinned huge page: it expects page count to be equal to
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not only in the head pages, if we want to be able to run
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sum of mapcount of all sub-pages plus one (split_huge_page caller must
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split_huge_page while there are gup pins established on any tail
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have reference for head page).
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page. Failure to be able to run split_huge_page if there's any gup pin
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on any tail page, would mean having to split all hugepages upfront in
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split_huge_page uses migration entries to stabilize page->_count and
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get_user_pages which is unacceptable as too many gup users are
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page->_mapcount.
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performance critical and they must work natively on hugepages like
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they work natively on hugetlbfs already (hugetlbfs is simpler because
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We safe against physical memory scanners too: the only legitimate way
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hugetlbfs pages cannot be split so there wouldn't be requirement of
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scanner can get reference to a page is get_page_unless_zero().
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accounting the pins on the tail pages for hugetlbfs). If we wouldn't
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account the gup refcounts on the tail pages during gup, we won't know
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All tail pages has zero ->_count until atomic_add(). It prevent scanner
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anymore which tail page is pinned by gup and which is not while we run
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from geting reference to tail page up to the point. After the atomic_add()
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split_huge_page. But we still have to add the gup pin to the head page
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we don't care about ->_count value. We already known how many references
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too, to know when we can free the compound page in case it's never
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with should uncharge from head page.
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split during its lifetime. That requires changing not just
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get_page, but put_page as well so that when put_page runs on a tail
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For head page get_page_unless_zero() will succeed and we don't mind. It's
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page (and only on a tail page) it will find its respective head page,
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clear where reference should go after split: it will stay on head page.
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and then it will decrease the head page refcount in addition to the
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tail page refcount. To obtain a head page reliably and to decrease its
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Note that split_huge_pmd() doesn't have any limitation on refcounting:
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refcount without race conditions, put_page has to serialize against
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pmd can be split at any point and never fails.
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__split_huge_page_refcount using a special per-page lock called
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compound_lock.
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== Partial unmap and deferred_split_huge_page() ==
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Unmapping part of THP (with munmap() or other way) is not going to free
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memory immediately. Instead, we detect that a subpage of THP is not in use
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in page_remove_rmap() and queue the THP for splitting if memory pressure
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comes. Splitting will free up unused subpages.
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Splitting the page right away is not an option due to locking context in
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the place where we can detect partial unmap. It's also might be
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counterproductive since in many cases partial unmap unmap happens during
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exit(2) if an THP crosses VMA boundary.
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Function deferred_split_huge_page() is used to queue page for splitting.
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The splitting itself will happen when we get memory pressure via shrinker
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interface.
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