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SubjectRe: [PATCH] mm/shmem: Do not wait for lock_page() in shmem_unused_huge_shrink()
On Fri 16-03-18 15:25:08, Kirill A. Shutemov wrote:
> On Fri, Mar 16, 2018 at 01:13:03PM +0100, Michal Hocko wrote:
> > On Fri 16-03-18 13:59:08, Kirill A. Shutemov wrote:
> > [..]
> > > @@ -498,31 +498,42 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
> > > continue;
> > > }
> > >
> > > - page = find_lock_page(inode->i_mapping,
> > > + page = find_get_page(inode->i_mapping,
> > > (inode->i_size & HPAGE_PMD_MASK) >> PAGE_SHIFT);
> > > if (!page)
> > > goto drop;
> > >
> > > + /* No huge page at the end of the file: nothing to split */
> > > if (!PageTransHuge(page)) {
> > > - unlock_page(page);
> > > put_page(page);
> > > goto drop;
> > > }
> > >
> > > + /*
> > > + * Leave the inode on the list if we failed to lock
> > > + * the page at this time.
> > > + *
> > > + * Waiting for the lock may lead to deadlock in the
> > > + * reclaim path.
> > > + */
> > > + if (!trylock_page(page)) {
> > > + put_page(page);
> > > + goto leave;
> > > + }
> >
> > Can somebody split the huge page after the PageTransHuge check and
> > before we lock it?
>
> Nope. Pin on the page is enough to prevent split.

Good, I thought so but wasn't really 100% sure. Thanks for the
clarification and feel free to add
Acked-by: Michal Hocko <mhocko@suse.com>

Maybe you should stick
Reported-by: Eric Wheeler <linux-mm@lists.ewheeler.net>
and point to http://lkml.kernel.org/r/alpine.LRH.2.11.1801242349220.30642@mail.ewheeler.net
because that smells like a bug that this patch would be fixing.
--
Michal Hocko
SUSE Labs

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