Re: [PATCH linux-next v3] mm: huge_memory: fix misused mapping_large_folio_support() for anon folios

[Andrew Morton]

On Fri, 7 Jun 2024 17:40:48 +0800 (CST) <xu.xin16@xxxxxxxxxx> wrote:

> From: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
> 
> 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.
> 

Can we pleae identify a Fixes: target for this?  Is it c010d47f107f
("mm: thp: split huge page to any lower order pages")?

It would be good to add a selftest which would have caught this.