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From: ran xiaokai <ranxiaokai627@163.com>
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Subject: Re: [PATCH linux-next v2] mm: huge_memory: fix misused mapping_large_folio_support() for anon folios
Date: Fri,  7 Jun 2024 01:36:17 +0000
Message-Id: <20240607013617.913054-1-ranxiaokai627@163.com>
In-Reply-To: <CAGsJ_4x=v0fDN_QVjdHSGVykH2+o_f81NnN_0-SUL+iwe+v84g@mail.gmail.com>
References: <CAGsJ_4x=v0fDN_QVjdHSGVykH2+o_f81NnN_0-SUL+iwe+v84g@mail.gmail.com>
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> > 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(-)
> >
> > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> > index ee633712bba0..59f1df0cde5a 100644
> > --- a/include/linux/pagemap.h
> > +++ b/include/linux/pagemap.h
> > @@ -381,6 +381,10 @@ static inline void mapping_set_large_folios(struct address_space *mapping)
> >   */
> >  static inline bool mapping_large_folio_support(struct address_space *mapping)
> >  {
> > +       /* AS_LARGE_FOLIO_SUPPORT is only reasonable for pagecache folios */
> > +       VM_WARN_ONCE((unsigned long)mapping & PAGE_MAPPING_ANON,
> > +                       "Anonymous mapping always supports large folio");
> > +
> >         return IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
> >                 test_bit(AS_LARGE_FOLIO_SUPPORT, &mapping->flags);
> >  }
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 317de2afd371..62d57270b08e 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -3009,30 +3009,35 @@ 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))
> > +       if (folio_test_anon(folio)) {
> > +               /* Cannot split anonymous THP to order-1 */
> 
> This is simply what the code is indicating. Shouldn't we phrase
> it differently to explain "why" but not "how"? for example, anon
> order-1 mTHP is not supported?

Hi, Barry,
Good comments, thanks.
Is "order-1 is not a anonymouns mTHP suitable order." better?
 
> Otherwise, it looks good to me.
> 
> Reviewed-by: Barry Song <baohua@kernel.org>