linux-kernel - Re: [PATCHv3 13/41] truncate: make sure invalidate_mapping

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Message-ID: <20161011215349.GC27110@node.shutemov.name>
Date:   Wed, 12 Oct 2016 00:53:49 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Jan Kara <jack@...e.cz>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-block@...r.kernel.org
Subject: Re: [PATCHv3 13/41] truncate: make sure invalidate_mapping_pages()
 can discard huge pages

On Tue, Oct 11, 2016 at 05:58:15PM +0200, Jan Kara wrote:
> On Thu 15-09-16 14:54:55, Kirill A. Shutemov wrote:
> > invalidate_inode_page() has expectation about page_count() of the page
> > -- if it's not 2 (one to caller, one to radix-tree), it will not be
> > dropped. That condition almost never met for THPs -- tail pages are
> > pinned to the pagevec.
> > 
> > Let's drop them, before calling invalidate_inode_page().
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >  mm/truncate.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/mm/truncate.c b/mm/truncate.c
> > index a01cce450a26..ce904e4b1708 100644
> > --- a/mm/truncate.c
> > +++ b/mm/truncate.c
> > @@ -504,10 +504,21 @@ unsigned long invalidate_mapping_pages(struct address_space *mapping,
> >  				/* 'end' is in the middle of THP */
> >  				if (index ==  round_down(end, HPAGE_PMD_NR))
> >  					continue;
> > +				/*
> > +				 * invalidate_inode_page() expects
> > +				 * page_count(page) == 2 to drop page from page
> > +				 * cache -- drop tail pages references.
> > +				 */
> > +				get_page(page);
> > +				pagevec_release(&pvec);
> 
> I'm not quite sure why this is needed. When you have multiorder entry in
> the radix tree for your huge page, then you should not get more entries in
> the pagevec for your huge page. What do I miss?

For compatibility reason find_get_entries() (which is called by
pagevec_lookup_entries()) collects all subpages of huge page in the range
(head/tails). See patch [07/41]

So huge page, which is fully in the range it will be pinned up to
PAGEVEC_SIZE times.

-- 
 Kirill A. Shutemov