linux-kernel - Re: [PATCH linux-next v3] mm: huge_memory: fix misused mapping_large_folio

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Date: Wed, 12 Jun 2024 04:43:33 +0000
From: ran xiaokai <ranxiaokai627@....com>
To: akpm@...ux-foundation.org
Cc: david@...hat.com,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	mhocko@...nel.org,
	ran.xiaokai@....com.cn,
	si.hao@....com.cn,
	v-songbaohua@...o.com,
	xu.xin16@....com.cn,
	yang.yang29@....com.cn,
	ziy@...dia.com
Subject: Re: [PATCH linux-next v3] mm: huge_memory: fix misused mapping_large_folio_support()  for anon folios

> > 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 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")?

yes, this fixes c010d47f107f ("mm: thp: split huge page to any lower order pages").

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

I have updated the code in selftests/mm/split_huge_page_test.c.
For now, only order-0 is tested for the anonymous THP split case,
I am adding more mTHP-suitable-orders test cases.
I will send that in a separate patch when it is done.