[CC Kirill]
On Mon 21-05-18 12:38:53, Song Liu wrote:
> khugepaged_enter_vma_merge() is using a different approach to check
> whether a vma is valid for khugepaged_enter():
>
> if (!vma->anon_vma)
> /*
> * Not yet faulted in so we will register later in the
> * page fault if needed.
> */
> return 0;
> if (vma->vm_ops || (vm_flags & VM_NO_KHUGEPAGED))
> /* khugepaged not yet working on file or special mappings */
> return 0;
>
> This check has some problems. One of the obvious problems is that
> it doesn't check shmem_file(), so that vma backed with shmem files
> will not call khugepaged_enter().
>
> This patch fixes these problems by reusing hugepage_vma_check() in
> khugepaged_enter_vma_merge().
It would be great to be more explicit about what are the actual
consequences. khugepaged_enter_vma_merge is called from multiple
context. Some of then do not really care about !anon case (e.g. stack
expansion). hugepage_madvise is quite convoluted so I am not really sure
from a quick look (are we simply not going to merge vmas even if we
could?).
Have you noticed this by a code inspection or you have seen this
happening in real workloads (aka, is this worth backporting to stable
trees)?
> Signed-off-by: Song Liu <songliubraving@xxxxxx>
> ---
> mm/khugepaged.c | 12 ++++--------
> 1 file changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index d7b2a4b..e50c2bd 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -430,18 +430,14 @@ int __khugepaged_enter(struct mm_struct *mm)
> return 0;
> }
>
> +static bool hugepage_vma_check(struct vm_area_struct *vma);
> +
> int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
> unsigned long vm_flags)
> {
> unsigned long hstart, hend;
> - if (!vma->anon_vma)
> - /*
> - * Not yet faulted in so we will register later in the
> - * page fault if needed.
> - */
> - return 0;
> - if (vma->vm_ops || (vm_flags & VM_NO_KHUGEPAGED))
> - /* khugepaged not yet working on file or special mappings */
> +
> + if (!hugepage_vma_check(vma))
> return 0;
> hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
> hend = vma->vm_end & HPAGE_PMD_MASK;
> --
> 2.9.5
--
Michal Hocko
SUSE Labs
