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SubjectRe: [PATCH] mm/shmem: Do not wait for lock_page() in shmem_unused_huge_shrink()
On Fri, Mar 16, 2018 at 01:58:27PM +0100, Michal Hocko wrote:
> 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>

Thanks.

> 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.

Good point.

Andrew, do you want me repost with tags integrated?

--
Kirill A. Shutemov

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