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

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Message-ID: <20170209201416.GS2267@bombadil.infradead.org>
Date:   Thu, 9 Feb 2017 12:14:16 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     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, 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@...rosoft.com>