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mm: mremap: downgrade mmap_sem to read when shrinking

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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.

[[email protected]: tweak comment]
[[email protected]: fix unsigned compare against 0 issue]
  Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Yang Shi <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Laurent Dufour <[email protected]>
Cc: Colin Ian King <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Yang Shi 2018-10-26 15:08:50 -07:00 • committed by Linus Torvalds
commit 85a06835f6
3 changed files with 20 additions and 6 deletions
Download patch file Download diff file Expand all files Collapse all files

4
mm/mmap.c
View file

@ -2687,8 +2687,8 @@ int split_vma(struct mm_struct *mm, struct vm_area_struct *vma,
* work. This now handles partial unmappings.
* Jeremy Fitzhardinge <jeremy@goop.org>
*/
static int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
struct list_head *uf, bool downgrade)
int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
struct list_head *uf, bool downgrade)
{
unsigned long end;
struct vm_area_struct *vma, *prev, *last;

20
mm/mremap.c
View file

@ -521,6 +521,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
unsigned long ret = -EINVAL;
unsigned long charged = 0;
bool locked = false;
bool downgraded = false;
struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
LIST_HEAD(uf_unmap_early);
LIST_HEAD(uf_unmap);
@ -557,12 +558,20 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
/*
* Always allow a shrinking remap: that just unmaps
* the unnecessary pages..
* do_munmap does all the needed commit accounting
* __do_munmap does all the needed commit accounting, and
* downgrades mmap_sem to read if so directed.
*/
if (old_len >= new_len) {
ret = do_munmap(mm, addr+new_len, old_len - new_len, &uf_unmap);
if (ret && old_len != new_len)
int retval;
retval = __do_munmap(mm, addr+new_len, old_len - new_len,
&uf_unmap, true);
if (retval < 0 && old_len != new_len) {
ret = retval;
goto out;
/* Returning 1 indicates mmap_sem is downgraded to read. */
} else if (retval == 1)
downgraded = true;
ret = addr;
goto out;
}
@ -627,7 +636,10 @@ out:
vm_unacct_memory(charged);
locked = 0;
}
up_write(&current->mm->mmap_sem);
if (downgraded)
up_read(&current->mm->mmap_sem);
else
up_write(&current->mm->mmap_sem);
if (locked && new_len > old_len)
mm_populate(new_addr + old_len, new_len - old_len);
userfaultfd_unmap_complete(mm, &uf_unmap_early);