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.
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)
>
