brk might be used to shinrk 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.
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
v2: Rephrase the commit per Michal
mm/mmap.c | 40 ++++++++++++++++++++++++++++++----------
1 file changed, 30 insertions(+), 10 deletions(-)
diff --git a/mm/mmap.c b/mm/mmap.c
index 017bcfa..0d2fae1 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -193,9 +193,11 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
unsigned long retval;
unsigned long newbrk, oldbrk;
struct mm_struct *mm = current->mm;
+ unsigned long origbrk = mm->brk;
struct vm_area_struct *next;
unsigned long min_brk;
bool populate;
+ bool downgrade = false;
LIST_HEAD(uf);
if (down_write_killable(&mm->mmap_sem))
@@ -229,14 +231,29 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
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;
+ /*
+ * mm->brk need to be protected by write mmap_sem, update it
+ * before downgrading mmap_sem.
+ * When __do_munmap fail, it will be restored from origbrk.
+ */
+ mm->brk = brk;
+ retval = __do_munmap(mm, newbrk, oldbrk-newbrk, &uf, true);
+ if (retval < 0) {
+ mm->brk = origbrk;
+ goto out;
+ } else if (retval == 1)
+ downgrade = true;
+ goto success;
}
/* Check against existing mmap mappings. */
@@ -247,18 +264,21 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
/* 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 (downgrade)
+ 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;
}
--
1.8.3.1
