Re: [PATCH linux-next v2] mm: huge_memory: fix misused mapping_large_folio_support() for anon folios

[Zi Yan <ziy@nvidia.com>]

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On 6 Jun 2024, at 2:42, xu.xin16@zte.com.cn wrote:

> From: Ran Xiaokai <ran.xiaokai@zte.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 the 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.
>
> Also add a VM_WARN_ON_ONCE() in mapping_large_folio_support()
> for anon mapping, So we can detect the wrong use more easily.
>
> THP folios maybe exist in the pagecache even the file system doesn't
> support large folio, it is because when CONFIG_TRANSPARENT_HUGEPAGE
> is enabled, khugepaged will try to collapse read-only file-backed pages
> to THP. But the mapping does not actually support multi order
> large folios properly.
>
> 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@zte.com.cn>
> ---
>  include/linux/pagemap.h |  4 ++++
>  mm/huge_memory.c        | 27 ++++++++++++++++-----------
>  2 files changed, 20 insertions(+), 11 deletions(-)

Reviewed-by: Zi Yan <ziy@nvidia.com>

Thanks.

Best Regards,
Yan, Zi

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