A-Team_Digital_Solutions/android_kernel_motorola_sm6375
Watch
2
0
Fork
You've already forked android_kernel_motorola_sm6375
 0
mirror of https://github.com/BobTheBlinker/android_kernel_motorola_sm6375.git synced 2026-10-28 10:41:38 -04:00
Code Issues Projects Releases Packages Wiki Activity

mm: brk: downgrade mmap_sem to read when shrinking

Browse source 
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.

[[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 15:08:54 -07:00 • committed by Linus Torvalds
commit 9bc8039e71
1 changed files with 35 additions and 11 deletions
Download patch file Download diff file Expand all files Collapse all files

46
mm/mmap.c
View file

@ -191,16 +191,19 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
SYSCALL_DEFINE1(brk, unsigned long, brk)
{
unsigned long retval;
unsigned long newbrk, oldbrk;
unsigned long newbrk, oldbrk, origbrk;
struct mm_struct *mm = current->mm;
struct vm_area_struct *next;
unsigned long min_brk;
bool populate;
bool downgraded = false;
LIST_HEAD(uf);
if (down_write_killable(&mm->mmap_sem))
return -EINTR;
origbrk = mm->brk;
#ifdef CONFIG_COMPAT_BRK
/*
* CONFIG_COMPAT_BRK can still be overridden by setting
@ -229,14 +232,32 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
newbrk = PAGE_ALIGN(brk);
oldbrk = PAGE_ALIGN(mm->brk);
if (oldbrk == newbrk)
goto set_brk;
if (oldbrk == newbrk) {
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 (!do_munmap(mm, newbrk, oldbrk-newbrk, &uf))
goto set_brk;
goto out;
int ret;
/*
* 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. */
@ -247,18 +268,21 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
/* Ok, looks good - let it rip. */
if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0)
goto out;
set_brk:
mm->brk = brk;
success:
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);
if (populate)
mm_populate(oldbrk, newbrk - oldbrk);
return brk;
out:
retval = mm->brk;
retval = origbrk;
up_write(&mm->mmap_sem);
return retval;
}