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Date:   Tue, 14 Apr 2020 23:54:29 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>
cc:     Randy Dunlap <rdunlap@infradead.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Linux Next Mailing List <linux-next@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>, Hugh Dickins <hughd@google.com>
Subject: Re: linux-next: Tree for Apr 14 (mm/shmem.c)
In-Reply-To: <20200414182430.c5af29ddb1735f5fd0083983@linux-foundation.org>
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References: <20200414123900.4f97a83f@canb.auug.org.au> <200c608a-8159-18ce-b44e-cad2022e23e2@infradead.org> <20200414182430.c5af29ddb1735f5fd0083983@linux-foundation.org>
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On Tue, 14 Apr 2020, Andrew Morton wrote:
> On Tue, 14 Apr 2020 07:18:01 -0700 Randy Dunlap <rdunlap@infradead.org> w=
rote:
> > On 4/13/20 7:39 PM, Stephen Rothwell wrote:
> > > Hi all,
> > >=20
> > > Changes since 20200413:
> > >=20
> > > New tree: mhi
> > >=20
> > > My fixes tree contains:
> > >=20
> > >   6b038bdcd3d1 sh: mm: Fix build error
> > >=20
> > > Non-merge commits (relative to Linus' tree): 1154
> > >  1160 files changed, 31764 insertions(+), 13498 deletions(-)
> > >=20
> > > ---------------------------------------------------------------------=
-------
> >=20
> > on x86_64:
> > # CONFIG_TRANSPARENT_HUGEPAGE is not set
>=20
> Thanks.  hm, this took a long time to be discovered.
>=20
> > In file included from ../include/linux/export.h:43:0,
> >                  from ../include/linux/linkage.h:7,
> >                  from ../include/linux/fs.h:5,
> >                  from ../mm/shmem.c:24:
> > ../mm/shmem.c: In function =E2=80=98shmem_undo_range=E2=80=99:
=2E..
> > ../mm/shmem.c:961:26: note: in expansion of macro =E2=80=98HPAGE_PMD_NR=
=E2=80=99
> >           round_up(start, HPAGE_PMD_NR))
> >                           ^~~~~~~~~~~~
>=20
> That's
> =09=09=09=09=09if (index <
> =09=09=09=09=09    round_up(start, HPAGE_PMD_NR))
> =09=09=09=09=09=09start =3D index + 1;
>=20
> from Hugh's 71725ed10c40696 ("mm: huge tmpfs: try to split_huge_page()
> when punching hole").

Sorry about that.  Yes, odd that it should only hit now: the false
PageTransCompound in shmem_punch_compound() has always been good
enough to handle it for me, but maybe Randy is trying a less able
compiler, or maybe unrelated changes in linux-next have just made
it harder for the compiler to see the optimization.

I hope the patch below fixes it?

[PATCH] mm/shmem: fix build without THP

Some optimizers don't notice that shmem_punch_compound() is always true
(PageTransCompound() being false) without CONFIG_TRANSPARENT_HUGEPAGE=3Dy:
use IS_ENABLED to help them to avoid the BUILD_BUG inside HPAGE_PMD_NR.

Fixes: 71725ed10c40 ("mm: huge tmpfs: try to split_huge_page() when punchin=
g hole")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Hugh Dickins <hughd@google.com>
---

 mm/shmem.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 5.7-rc1/mm/shmem.c=092020-04-11 12:58:26.415524805 -0700
+++ linux/mm/shmem.c=092020-04-14 23:20:25.517656174 -0700
@@ -952,7 +952,7 @@ static void shmem_undo_range(struct inod
 =09=09=09=09VM_BUG_ON_PAGE(PageWriteback(page), page);
 =09=09=09=09if (shmem_punch_compound(page, start, end))
 =09=09=09=09=09truncate_inode_page(mapping, page);
-=09=09=09=09else {
+=09=09=09=09else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
 =09=09=09=09=09/* Wipe the page and don't get stuck */
 =09=09=09=09=09clear_highpage(page);
 =09=09=09=09=09flush_dcache_page(page);
--0-6116103-1586933689=:10035--