linux-kernel - Re: [PATCHv6 05/37] thp: try to free page's buffers before attempt split

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Date:   Mon, 13 Feb 2017 17:32:47 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Matthew Wilcox <willy@...radead.org>
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>,
        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: [PATCHv6 05/37] thp: try to free page's buffers before attempt
 split

On Thu, Feb 09, 2017 at 12:14:16PM -0800, Matthew Wilcox wrote:
> On Thu, Jan 26, 2017 at 02:57:47PM +0300, Kirill A. Shutemov wrote:
> > @@ -2146,6 +2146,23 @@ 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.
> > +			 */
> 
> It took me a few reads to get this.  May I suggest:
> 
> 		/*
> 		 * Cannot split a page with buffers.  If the caller has
> 		 * already started writeback, we can wait for it to finish,
> 		 * but we cannot start writeback if we were called from vmscan
> 		 */

Yeah, that's better.

> > +		if (!PageSwapBacked(head) && PagePrivate(page)) {
> 
> Also, it looks weird to test PageSwapBacked of *head* and PagePrivate
> of *page*.  I think it's correct, but it still looks weird.

I'll change this.

> Reviewed-by: Matthew Wilcox <mawilcox@...rosoft.com>

Thanks!

-- 
 Kirill A. Shutemov