linux-kernel - Re: [PATCH] mm/shmem: Do not wait for lock_page() in shmem_unused_huge

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Date:   Fri, 16 Mar 2018 13:58:27 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Eric Wheeler <linux-mm@...ts.ewheeler.net>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Subject: Re: [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@...e.com>

Maybe you should stick
Reported-by: Eric Wheeler <linux-mm@...ts.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