On 4/24/19 6:10 AM, Vlastimil Babka wrote:
On 4/23/19 6:43 PM, Yang Shi wrote:
The commit 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each
vma") introduced THPeligible bit for processes' smaps. But, when checking
the eligibility for shmem vma, __transparent_hugepage_enabled() is
called to override the result from shmem_huge_enabled(). It may result
in the anonymous vma's THP flag override shmem's. For example, running a
simple test which create THP for shmem, but with anonymous THP disabled,
when reading the process's smaps, it may show:
7fc92ec00000-7fc92f000000 rw-s 00000000 00:14 27764 /dev/shm/test
Size: 4096 kB
...
[snip]
...
ShmemPmdMapped: 4096 kB
But how does this happen in the first place?
In __handle_mm_fault() we do:
if (pmd_none(*vmf.pmd) && __transparent_hugepage_enabled(vma)) {
ret = create_huge_pmd(&vmf);
if (!(ret & VM_FAULT_FALLBACK))
return ret;
And __transparent_hugepage_enabled() checks the global THP settings.
If THP is not enabled / is only for madvise and the vma is not madvised,
then this should fail, and also khugepaged shouldn't either run at all,
or don't do its job for such non-madvised vma.
If __transparent_hugepage_enabled() returns false, the code will not
reach create_huge_pmd() at all. If it returns true, create_huge_pmd()
actually will return VM_FAULT_FALLBACK for shmem since shmem doesn't
have huge_fault (or pmd_fault in earlier versions) method.
Then it will get into handle_pte_fault(), finally shmem_fault() is
called, which allocates THP by checking some global flag (i.e.
VM_NOHUGEPAGE and MMF_DISABLE_THP) and shmem THP knobs.
4.8 (the first version has shmem THP merged) behaves exactly in the same
way. So, I suspect this may be intended behavior.
What am I missing?
...
[snip]
...
THPeligible: 0
And, /proc/meminfo does show THP allocated and PMD mapped too:
ShmemHugePages: 4096 kB
ShmemPmdMapped: 4096 kB
This doesn't make too much sense. The anonymous THP flag should not
intervene shmem THP. Calling shmem_huge_enabled() with checking
MMF_DISABLE_THP sounds good enough. And, we could skip stack and
dax vma check since we already checked if the vma is shmem already.
Fixes: 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each vma")
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
v2: Check VM_NOHUGEPAGE per Michal Hocko
mm/huge_memory.c | 4 ++--
mm/shmem.c | 3 +++
2 files changed, 5 insertions(+), 2 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 165ea46..5881e82 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -67,8 +67,8 @@ bool transparent_hugepage_enabled(struct vm_area_struct *vma)
{
if (vma_is_anonymous(vma))
return __transparent_hugepage_enabled(vma);
- if (vma_is_shmem(vma) && shmem_huge_enabled(vma))
- return __transparent_hugepage_enabled(vma);
+ if (vma_is_shmem(vma))
+ return shmem_huge_enabled(vma);
return false;
}
diff --git a/mm/shmem.c b/mm/shmem.c
index 2275a0f..6f09a31 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3873,6 +3873,9 @@ bool shmem_huge_enabled(struct vm_area_struct *vma)
loff_t i_size;
pgoff_t off;
+ if ((vma->vm_flags & VM_NOHUGEPAGE) ||
+ test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
+ return false;
if (shmem_huge == SHMEM_HUGE_FORCE)
return true;
if (shmem_huge == SHMEM_HUGE_DENY)
