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Date:   Wed, 15 Apr 2020 08:34:27 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: linux-next: Tree for Apr 14 (mm/shmem.c)

On 4/14/20 11:54 PM, Hugh Dickins wrote:
> On Tue, 14 Apr 2020, Andrew Morton wrote:
>> On Tue, 14 Apr 2020 07:18:01 -0700 Randy Dunlap <rdunlap@...radead.org> wrote:
>>> On 4/13/20 7:39 PM, Stephen Rothwell wrote:
>>>> Hi all,
>>>>
>>>> Changes since 20200413:
>>>>
>>>> New tree: mhi
>>>>
>>>> My fixes tree contains:
>>>>
>>>>   6b038bdcd3d1 sh: mm: Fix build error
>>>>
>>>> Non-merge commits (relative to Linus' tree): 1154
>>>>  1160 files changed, 31764 insertions(+), 13498 deletions(-)
>>>>
>>>> ----------------------------------------------------------------------------
>>>
>>> on x86_64:
>>> # CONFIG_TRANSPARENT_HUGEPAGE is not set
>>
>> Thanks.  hm, this took a long time to be discovered.
>>
>>> 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 ‘shmem_undo_range’:
> ...
>>> ../mm/shmem.c:961:26: note: in expansion of macro ‘HPAGE_PMD_NR’
>>>           round_up(start, HPAGE_PMD_NR))
>>>                           ^~~~~~~~~~~~
>>
>> That's
>> 					if (index <
>> 					    round_up(start, HPAGE_PMD_NR))
>> 						start = index + 1;
>>
>> 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=y:
> 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 punching hole")
> Reported-by: Randy Dunlap <rdunlap@...radead.org>
> Signed-off-by: Hugh Dickins <hughd@...gle.com>

Works for me, using gcc 7.5.0.

Acked-by: Randy Dunlap <rdunlap@...radead.org> # build-tested

Thanks.

> ---
> 
>  mm/shmem.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- 5.7-rc1/mm/shmem.c	2020-04-11 12:58:26.415524805 -0700
> +++ linux/mm/shmem.c	2020-04-14 23:20:25.517656174 -0700
> @@ -952,7 +952,7 @@ static void shmem_undo_range(struct inod
>  				VM_BUG_ON_PAGE(PageWriteback(page), page);
>  				if (shmem_punch_compound(page, start, end))
>  					truncate_inode_page(mapping, page);
> -				else {
> +				else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
>  					/* Wipe the page and don't get stuck */
>  					clear_highpage(page);
>  					flush_dcache_page(page);
> 


-- 
~Randy

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