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Date: Thu, 9 Feb 2017 12:14:16 -0800
From: Matthew Wilcox <willy@infradead.org>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Theodore Ts'o <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jan Kara <jack@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Hugh Dickins <hughd@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-block@vger.kernel.org
Subject: Re: [PATCHv6 05/37] thp: try to free page's buffers before attempt
 split
Message-ID: <20170209201416.GS2267@bombadil.infradead.org>
References: <20170126115819.58875-1-kirill.shutemov@linux.intel.com>
 <20170126115819.58875-6-kirill.shutemov@linux.intel.com>
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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
		 */
> +		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.

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