mm: brk: downgrade mmap_sem to read when shrinking
brk might be used to shrink memory mapping too other than munmap(). So, it may hold write mmap_sem for long time when shrinking large mapping, as what commit ("mm: mmap: zap pages with read mmap_sem in munmap") described. The brk() will not manipulate vmas anymore after __do_munmap() call for the mapping shrink use case. But, it may set mm->brk after __do_munmap(), which needs hold write mmap_sem. However, a simple trick can workaround this by setting mm->brk before __do_munmap(). Then restore the original value if __do_munmap() fails. With this trick, it is safe to downgrade to read mmap_sem. So, the same optimization, which downgrades mmap_sem to read for zapping pages, is also feasible and reasonable to this case. The period of holding exclusive mmap_sem for shrinking large mapping would be reduced significantly with this optimization. [akpm@linux-foundation.org: tweak comment] [yang.shi@linux.alibaba.com: fix unsigned compare against 0 issue] Link: http://lkml.kernel.org/r/1538687672-17795-1-git-send-email-yang.shi@linux.alibaba.com Link: http://lkml.kernel.org/r/1538067582-60038-2-git-send-email-yang.shi@linux.alibaba.com Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com> Acked-by: Vlastimil Babka <vbabka@suse.cz> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Matthew Wilcox <willy@infradead.org> Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com> Cc: Colin Ian King <colin.king@canonical.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
parent
85a06835f6
commit
9bc8039e71
42
mm/mmap.c
42
mm/mmap.c
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@ -191,16 +191,19 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
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SYSCALL_DEFINE1(brk, unsigned long, brk)
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{
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unsigned long retval;
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unsigned long newbrk, oldbrk;
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unsigned long newbrk, oldbrk, origbrk;
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struct mm_struct *mm = current->mm;
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struct vm_area_struct *next;
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unsigned long min_brk;
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bool populate;
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bool downgraded = false;
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LIST_HEAD(uf);
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if (down_write_killable(&mm->mmap_sem))
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return -EINTR;
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origbrk = mm->brk;
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#ifdef CONFIG_COMPAT_BRK
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/*
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* CONFIG_COMPAT_BRK can still be overridden by setting
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@ -229,14 +232,32 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
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newbrk = PAGE_ALIGN(brk);
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oldbrk = PAGE_ALIGN(mm->brk);
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if (oldbrk == newbrk)
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goto set_brk;
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if (oldbrk == newbrk) {
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mm->brk = brk;
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goto success;
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}
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/* Always allow shrinking brk. */
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/*
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* Always allow shrinking brk.
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* __do_munmap() may downgrade mmap_sem to read.
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*/
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if (brk <= mm->brk) {
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if (!do_munmap(mm, newbrk, oldbrk-newbrk, &uf))
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goto set_brk;
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int ret;
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/*
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* mm->brk must to be protected by write mmap_sem so update it
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* before downgrading mmap_sem. When __do_munmap() fails,
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* mm->brk will be restored from origbrk.
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*/
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mm->brk = brk;
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ret = __do_munmap(mm, newbrk, oldbrk-newbrk, &uf, true);
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if (ret < 0) {
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mm->brk = origbrk;
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goto out;
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} else if (ret == 1) {
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downgraded = true;
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}
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goto success;
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}
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/* Check against existing mmap mappings. */
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@ -247,10 +268,13 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
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/* Ok, looks good - let it rip. */
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if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0)
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goto out;
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set_brk:
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mm->brk = brk;
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success:
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populate = newbrk > oldbrk && (mm->def_flags & VM_LOCKED) != 0;
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if (downgraded)
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up_read(&mm->mmap_sem);
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else
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up_write(&mm->mmap_sem);
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userfaultfd_unmap_complete(mm, &uf);
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if (populate)
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@ -258,7 +282,7 @@ set_brk:
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return brk;
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out:
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retval = mm->brk;
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retval = origbrk;
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up_write(&mm->mmap_sem);
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return retval;
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}
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