linux-kernel - Re: [PATCH linux-next] mm: huge_memory: fix misused mapping_large_folio

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Date: Wed, 5 Jun 2024 21:06:57 +1200
From: Barry Song <21cnbao@...il.com>
To: xu.xin16@....com.cn
Cc: akpm@...ux-foundation.org, ziy@...dia.com, v-songbaohua@...o.com, 
	mhocko@...nel.org, david@...hat.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, yang.yang29@....com.cn
Subject: Re: [PATCH linux-next] mm: huge_memory: fix misused
 mapping_large_folio_support() for anon folios

On Tue, Jun 4, 2024 at 5:47 PM <xu.xin16@....com.cn> wrote:
>
> From: Ran Xiaokai <ran.xiaokai@....com.cn>
>
> When I did a large folios split test, a WARNING
> "[ 5059.122759][  T166] Cannot split file folio to non-0 order"
> was triggered. But my test cases are only for anonmous folios.
> while mapping_large_folio_support() is only reasonable for page
> cache folios.
>
> In split_huge_page_to_list_to_order(), the folio passed to
> mapping_large_folio_support() maybe anonmous folio. The
> folio_test_anon() check is missing. So the split of the anonmous THP
> is failed. This is also the same for shmem_mapping(). We'd better add
> a check for both. But the shmem_mapping() in __split_huge_page() is
> not involved, as for anonmous folios, the end parameter is set to -1, so
> (head[i].index >= end) is always false. shmem_mapping() is not called.
>
> Using /sys/kernel/debug/split_huge_pages to verify this, with this
> patch, large anon THP is successfully split and the warning is ceased.
>
> Signed-off-by: Ran Xiaokai <ran.xiaokai@....com.cn>
> Cc: xu xin <xu.xin16@....com.cn>
> Cc: Yang Yang <yang.yang29@....com.cn>
> ---
>  mm/huge_memory.c | 38 ++++++++++++++++++++------------------
>  1 file changed, 20 insertions(+), 18 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 317de2afd371..4c9c7e5ea20c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3009,31 +3009,33 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>         if (new_order >= folio_order(folio))
>                 return -EINVAL;
>
> -       /* Cannot split anonymous THP to order-1 */
> -       if (new_order == 1 && folio_test_anon(folio)) {
> -               VM_WARN_ONCE(1, "Cannot split to order-1 folio");
> -               return -EINVAL;
> -       }
> -
>         if (new_order) {
>                 /* Only swapping a whole PMD-mapped folio is supported */
>                 if (folio_test_swapcache(folio))
>                         return -EINVAL;
> -               /* Split shmem folio to non-zero order not supported */
> -               if (shmem_mapping(folio->mapping)) {
> -                       VM_WARN_ONCE(1,
> -                               "Cannot split shmem folio to non-0 order");
> -                       return -EINVAL;
> -               }
> -               /* No split if the file system does not support large folio */
> -               if (!mapping_large_folio_support(folio->mapping)) {
> -                       VM_WARN_ONCE(1,
> -                               "Cannot split file folio to non-0 order");
> -                       return -EINVAL;
> +
> +               if (folio_test_anon(folio)) {
> +                       /* Cannot split anonymous THP to order-1 */
> +                       if (new_order == 1) {
> +                               VM_WARN_ONCE(1, "Cannot split to order-1 folio");
> +                               return -EINVAL;
> +                       }
> +               } else {
> +                       /* Split shmem folio to non-zero order not supported */
> +                       if (shmem_mapping(folio->mapping)) {
> +                               VM_WARN_ONCE(1,
> +                                       "Cannot split shmem folio to non-0 order");
> +                               return -EINVAL;
> +                       }
> +                       /* No split if the file system does not support large folio */
> +                       if (!mapping_large_folio_support(folio->mapping)) {
> +                               VM_WARN_ONCE(1,
> +                                       "Cannot split file folio to non-0 order");
> +                               return -EINVAL;
> +                       }

Am I missing something? if file system doesn't support large folio,
how could the large folio start to exist from the first place while its
mapping points to a file which doesn't support large folio?

>                 }
>         }
>
> -
>         is_hzp = is_huge_zero_folio(folio);
>         if (is_hzp) {
>                 pr_warn_ratelimited("Called split_huge_page for huge zero page\n");
> --
> 2.15.2
>

Thanks
Barry