mm: mremap: downgrade mmap_sem to read when shrinking
Other than munmap, mremap might be used to shrink memory mapping too. 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 mremap() will not manipulate vmas anymore after __do_munmap() call for the mapping shrink use case, so 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. MREMAP_FIXED and MREMAP_MAYMOVE are more complicated to adopt this optimization since they need manipulate vmas after do_munmap(), downgrading mmap_sem may create race window. Simple mapping shrink is the low hanging fruit, and it may cover the most cases of unmap with munmap together. [akpm@linux-foundation.org: tweak comment] [yang.shi@linux.alibaba.com: fix unsigned compare against 0 issue] Link: http://lkml.kernel.org/r/1538687672-17795-2-git-send-email-yang.shi@linux.alibaba.com Link: http://lkml.kernel.org/r/1538067582-60038-1-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>
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@ -2306,6 +2306,8 @@ extern unsigned long do_mmap(struct file *file, unsigned long addr,
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unsigned long len, unsigned long prot, unsigned long flags,
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vm_flags_t vm_flags, unsigned long pgoff, unsigned long *populate,
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struct list_head *uf);
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extern int __do_munmap(struct mm_struct *, unsigned long, size_t,
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struct list_head *uf, bool downgrade);
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extern int do_munmap(struct mm_struct *, unsigned long, size_t,
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struct list_head *uf);
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@ -2687,7 +2687,7 @@ int split_vma(struct mm_struct *mm, struct vm_area_struct *vma,
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* work. This now handles partial unmappings.
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* Jeremy Fitzhardinge <jeremy@goop.org>
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*/
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static int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
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int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
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struct list_head *uf, bool downgrade)
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{
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unsigned long end;
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18
mm/mremap.c
18
mm/mremap.c
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@ -521,6 +521,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
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unsigned long ret = -EINVAL;
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unsigned long charged = 0;
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bool locked = false;
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bool downgraded = false;
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struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
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LIST_HEAD(uf_unmap_early);
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LIST_HEAD(uf_unmap);
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@ -557,12 +558,20 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
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/*
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* Always allow a shrinking remap: that just unmaps
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* the unnecessary pages..
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* do_munmap does all the needed commit accounting
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* __do_munmap does all the needed commit accounting, and
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* downgrades mmap_sem to read if so directed.
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*/
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if (old_len >= new_len) {
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ret = do_munmap(mm, addr+new_len, old_len - new_len, &uf_unmap);
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if (ret && old_len != new_len)
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int retval;
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retval = __do_munmap(mm, addr+new_len, old_len - new_len,
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&uf_unmap, true);
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if (retval < 0 && old_len != new_len) {
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ret = retval;
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goto out;
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/* Returning 1 indicates mmap_sem is downgraded to read. */
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} else if (retval == 1)
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downgraded = true;
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ret = addr;
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goto out;
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}
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@ -627,6 +636,9 @@ out:
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vm_unacct_memory(charged);
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locked = 0;
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}
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if (downgraded)
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up_read(¤t->mm->mmap_sem);
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else
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up_write(¤t->mm->mmap_sem);
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if (locked && new_len > old_len)
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mm_populate(new_addr + old_len, new_len - old_len);
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