linux-ext4 - Re: [PATCHv3 11/41] thp: try to free page's buffers before attempt split

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Date:   Wed, 12 Oct 2016 00:43:53 +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 11/41] thp: try to free page's buffers before attempt
 split

On Tue, Oct 11, 2016 at 05:40:31PM +0200, Jan Kara wrote:
> On Thu 15-09-16 14:54:53, Kirill A. Shutemov wrote:
> > We want page to be isolated from the rest of the system before spliting
> > it. We rely on page count to be 2 for file pages to make sure nobody
> > uses the page: one pin to caller, one to radix-tree.
> > 
> > Filesystems with backing storage can have page count increased if it has
> > buffers.
> > 
> > Let's try to free them, before attempt split. And remove one guarding
> > VM_BUG_ON_PAGE().
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ...
> > @@ -2041,6 +2041,23 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> >  			goto out;
> >  		}
> >  
> > +		/* Try to free buffers before attempt split */
> > +		if (!PageSwapBacked(head) && PagePrivate(page)) {
> > +			/*
> > +			 * We cannot trigger writeback from here due possible
> > +			 * recursion if triggered from vmscan, only wait.
> > +			 *
> > +			 * Caller can trigger writeback it on its own, if safe.
> > +			 */
> > +			wait_on_page_writeback(head);
> > +
> > +			if (page_has_buffers(head) &&
> > +					!try_to_free_buffers(head)) {
> > +				ret = -EBUSY;
> > +				goto out;
> > +			}
> 
> Shouldn't you rather use try_to_release_page() here? Because filesystems
> have their ->releasepage() callbacks for freeing data associated with a
> page. It is not guaranteed page private data are buffers although it is
> true for ext4...

Fair enough. Will fix this.

-- 
 Kirill A. Shutemov
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