Received-SPF: pass (google.com: domain of linux-kernel+bounces-205238-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.48.161 as permitted sender) client-ip=147.75.48.161;
Precedence: bulk
MIME-Version: 1.0
References: <CAGsJ_4x=v0fDN_QVjdHSGVykH2+o_f81NnN_0-SUL+iwe+v84g@mail.gmail.com>
 <20240607013617.913054-1-ranxiaokai627@163.com>
In-Reply-To: <20240607013617.913054-1-ranxiaokai627@163.com>
From: Barry Song <21cnbao@gmail.com>
Date: Fri, 7 Jun 2024 14:00:47 +1200
Message-ID: <CAGsJ_4yt2eegrB5fDsGMZiirzhqNtnRGOOnhvrvv-0V=x_WG=A@mail.gmail.com>
Subject: Re: [PATCH linux-next v2] mm: huge_memory: fix misused
 mapping_large_folio_support() for anon folios
To: ran xiaokai <ranxiaokai627@163.com>
Cc: akpm@linux-foundation.org, david@redhat.com, linux-kernel@vger.kernel.org, 
	linux-mm@kvack.org, mhocko@kernel.org, ran.xiaokai@zte.com.cn, 
	v-songbaohua@oppo.com, si.hao@zte.com.cn, xu.xin16@zte.com.cn, 
	yang.yang29@zte.com.cn, ziy@nvidia.com
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Fri, Jun 7, 2024 at 1:37=E2=80=AFPM ran xiaokai <ranxiaokai627@163.com> =
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 >=3D end) is always false. shmem_mapping() is not call=
ed.
> > >
> > > 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 pag=
es
> > > 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(stru=
ct address_space *mapping)
> > >   */
> > >  static inline bool mapping_large_folio_support(struct address_space =
*mapping)
> > >  {
> > > +       /* AS_LARGE_FOLIO_SUPPORT is only reasonable for pagecache fo=
lios */
> > > +       VM_WARN_ONCE((unsigned long)mapping & PAGE_MAPPING_ANON,
> > > +                       "Anonymous mapping always supports large foli=
o");
> > > +
> > >         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 p=
age *page, struct list_head *list,
> > >         if (new_order >=3D folio_order(folio))
> > >                 return -EINVAL;
> > >
> > > -       /* Cannot split anonymous THP to order-1 */
> > > -       if (new_order =3D=3D 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 supporte=
d */
> > > -               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?

could pick up some words from include/linux/huge_mm.h, particularly
those words regarding "a limitation of the THP implementation".

/*
 * Mask of all large folio orders supported for anonymous THP; all orders u=
p to
 * and including PMD_ORDER, except order-0 (which is not "huge") and order-=
1
 * (which is a limitation of the THP implementation).
 */
#define THP_ORDERS_ALL_ANON     ((BIT(PMD_ORDER + 1) - 1) & ~(BIT(0) | BIT(=
1)))

perhaps, you can even do

if (order > 0 && !(bit(order) & THP_ORDERS_ALL_ANON))
      return -EINVAL;

This is self-commented. Either way is fine.

>
> > Otherwise, it looks good to me.
> >
> > Reviewed-by: Barry Song <baohua@kernel.org>
>

Thanks
Barry