Re: [patch 1/2] mm, shmem: add vmstat for hugepage fallback

[Yang Shi <shy828301@gmail.com>]

On Fri, Mar 6, 2020 at 2:22 PM David Rientjes <rientjes@google.com> wrote:
>
> The existing thp_fault_fallback indicates when thp attempts to allocate a
> hugepage but fails, or if the hugepage cannot be charged to the mem cgroup
> hierarchy.
>
> Extend this to shmem as well.  Adds a new thp_file_fallback to complement
> thp_file_alloc that gets incremented when a hugepage is attempted to be
> allocated but fails, or if it cannot be charged to the mem cgroup
> hierarchy.
>
> Additionally, remove the check for CONFIG_TRANSPARENT_HUGE_PAGECACHE from
> shmem_alloc_hugepage() since it is only called with this configuration
> option.

Looks good to me. Thanks for taking this suggestion. Reviewed-by: Yang
Shi <yang.shi@linux.alibaba.com>

>
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst |  4 ++++
>  include/linux/vm_event_item.h              |  2 ++
>  mm/shmem.c                                 | 10 ++++++----
>  mm/vmstat.c                                |  1 +
>  4 files changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -319,6 +319,10 @@ thp_file_alloc
>         is incremented every time a file huge page is successfully
>         allocated.
>
> +thp_file_fallback
> +       is incremented if a file huge page is attempted to be allocated
> +       but fails and instead falls back to using small pages.
> +
>  thp_file_mapped
>         is incremented every time a file huge page is mapped into
>         user address space.
> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
> --- a/include/linux/vm_event_item.h
> +++ b/include/linux/vm_event_item.h
> @@ -76,6 +76,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>                 THP_COLLAPSE_ALLOC,
>                 THP_COLLAPSE_ALLOC_FAILED,
>                 THP_FILE_ALLOC,
> +               THP_FILE_FALLBACK,
>                 THP_FILE_MAPPED,
>                 THP_SPLIT_PAGE,
>                 THP_SPLIT_PAGE_FAILED,
> @@ -115,6 +116,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>
>  #ifndef CONFIG_TRANSPARENT_HUGEPAGE
>  #define THP_FILE_ALLOC ({ BUILD_BUG(); 0; })
> +#define THP_FILE_FALLBACK ({ BUILD_BUG(); 0; })
>  #define THP_FILE_MAPPED ({ BUILD_BUG(); 0; })
>  #endif
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1472,9 +1472,6 @@ static struct page *shmem_alloc_hugepage(gfp_t gfp,
>         pgoff_t hindex;
>         struct page *page;
>
> -       if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))
> -               return NULL;
> -
>         hindex = round_down(index, HPAGE_PMD_NR);
>         if (xa_find(&mapping->i_pages, &hindex, hindex + HPAGE_PMD_NR - 1,
>                                                                 XA_PRESENT))
> @@ -1486,6 +1483,8 @@ static struct page *shmem_alloc_hugepage(gfp_t gfp,
>         shmem_pseudo_vma_destroy(&pvma);
>         if (page)
>                 prep_transhuge_page(page);
> +       else
> +               count_vm_event(THP_FILE_FALLBACK);
>         return page;
>  }
>
> @@ -1871,8 +1870,11 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
>
>         error = mem_cgroup_try_charge_delay(page, charge_mm, gfp, &memcg,
>                                             PageTransHuge(page));
> -       if (error)
> +       if (error) {
> +               if (PageTransHuge(page))
> +                       count_vm_event(THP_FILE_FALLBACK);
>                 goto unacct;
> +       }
>         error = shmem_add_to_page_cache(page, mapping, hindex,
>                                         NULL, gfp & GFP_RECLAIM_MASK);
>         if (error) {
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1257,6 +1257,7 @@ const char * const vmstat_text[] = {
>         "thp_collapse_alloc",
>         "thp_collapse_alloc_failed",
>         "thp_file_alloc",
> +       "thp_file_fallback",
>         "thp_file_mapped",
>         "thp_split_page",
>         "thp_split_page_failed",