Commit 9bc8039e authored by Yang Shi's avatar Yang Shi Committed by Linus Torvalds

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.comSigned-off-by: default avatarYang Shi <yang.shi@linux.alibaba.com>
Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
Acked-by: default avatarKirill 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 85a06835
...@@ -191,16 +191,19 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long ...@@ -191,16 +191,19 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
SYSCALL_DEFINE1(brk, unsigned long, brk) SYSCALL_DEFINE1(brk, unsigned long, brk)
{ {
unsigned long retval; unsigned long retval;
unsigned long newbrk, oldbrk; unsigned long newbrk, oldbrk, origbrk;
struct mm_struct *mm = current->mm; struct mm_struct *mm = current->mm;
struct vm_area_struct *next; struct vm_area_struct *next;
unsigned long min_brk; unsigned long min_brk;
bool populate; bool populate;
bool downgraded = false;
LIST_HEAD(uf); LIST_HEAD(uf);
if (down_write_killable(&mm->mmap_sem)) if (down_write_killable(&mm->mmap_sem))
return -EINTR; return -EINTR;
origbrk = mm->brk;
#ifdef CONFIG_COMPAT_BRK #ifdef CONFIG_COMPAT_BRK
/* /*
* CONFIG_COMPAT_BRK can still be overridden by setting * CONFIG_COMPAT_BRK can still be overridden by setting
...@@ -229,14 +232,32 @@ SYSCALL_DEFINE1(brk, unsigned long, brk) ...@@ -229,14 +232,32 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
newbrk = PAGE_ALIGN(brk); newbrk = PAGE_ALIGN(brk);
oldbrk = PAGE_ALIGN(mm->brk); oldbrk = PAGE_ALIGN(mm->brk);
if (oldbrk == newbrk) if (oldbrk == newbrk) {
goto set_brk; mm->brk = brk;
goto success;
}
/* Always allow shrinking brk. */ /*
* Always allow shrinking brk.
* __do_munmap() may downgrade mmap_sem to read.
*/
if (brk <= mm->brk) { if (brk <= mm->brk) {
if (!do_munmap(mm, newbrk, oldbrk-newbrk, &uf)) int ret;
goto set_brk;
goto out; /*
* mm->brk must to be protected by write mmap_sem so update it
* before downgrading mmap_sem. When __do_munmap() fails,
* mm->brk will be restored from origbrk.
*/
mm->brk = brk;
ret = __do_munmap(mm, newbrk, oldbrk-newbrk, &uf, true);
if (ret < 0) {
mm->brk = origbrk;
goto out;
} else if (ret == 1) {
downgraded = true;
}
goto success;
} }
/* Check against existing mmap mappings. */ /* Check against existing mmap mappings. */
...@@ -247,18 +268,21 @@ SYSCALL_DEFINE1(brk, unsigned long, brk) ...@@ -247,18 +268,21 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
/* Ok, looks good - let it rip. */ /* Ok, looks good - let it rip. */
if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0) if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0)
goto out; goto out;
set_brk:
mm->brk = brk; mm->brk = brk;
success:
populate = newbrk > oldbrk && (mm->def_flags & VM_LOCKED) != 0; populate = newbrk > oldbrk && (mm->def_flags & VM_LOCKED) != 0;
up_write(&mm->mmap_sem); if (downgraded)
up_read(&mm->mmap_sem);
else
up_write(&mm->mmap_sem);
userfaultfd_unmap_complete(mm, &uf); userfaultfd_unmap_complete(mm, &uf);
if (populate) if (populate)
mm_populate(oldbrk, newbrk - oldbrk); mm_populate(oldbrk, newbrk - oldbrk);
return brk; return brk;
out: out:
retval = mm->brk; retval = origbrk;
up_write(&mm->mmap_sem); up_write(&mm->mmap_sem);
return retval; return retval;
} }
......
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