Re: [PATCH v2 2/2] smaps: set THPeligible if file mapping supports large folios

["Zach O'Keefe" <zokeefe@google.com>]

Sorry -- noticed only too late that there are still many
false-negatives for THPeligible, since by this point in the function
we've already applied sysfs and prctl restrictions, which file-fault
ignores. VM_HUGEPAGE also needs to be checked for the file-fault case.

On Fri, Aug 18, 2023 at 2:15 PM Zach O'Keefe <zokeefe@google.com> wrote:
>
> File-backed memory can be backed by THPs either through collapse, when
> CONFIG_READ_ONLY_THP_FOR_FS is enabled, or through fault, when the
> filesystem supports large folio mappings.
>
> Currently, smaps only knows about the former, so teach it about the
> latter.
>
> Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> ---
>  mm/huge_memory.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index cd379b2c077b..d8d6e83820f3 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -136,7 +136,16 @@ bool hugepage_vma_check(struct vm_area_struct *vma, unsigned long vm_flags,
>                          */
>                         !!vma->vm_ops->huge_fault :
>                         /* Only regular file is valid in collapse path */
> -                       file_thp_enabled(vma);
> +                       file_thp_enabled(vma) ||
> +                        /*
> +                         * THPeligible bit of smaps should surface the
> +                         * possibility of THP through fault if the filesystem
> +                         * supports it.  We don't check this in fault path,
> +                         * because we want to fallback to the actual ->fault()
> +                         * handler to make the decision.
> +                         */
> +                        (smaps && vma->vm_file &&
> +                        mapping_large_folio_support(vma->vm_file->f_mapping));
>
>         if (vma_is_temporary_stack(vma))
>                 return false;
> --
> 2.42.0.rc1.204.g551eb34607-goog
>