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

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Date:   Tue, 11 Oct 2016 17:40:31 +0200
From:   Jan Kara <jack@...e.cz>
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>,
        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 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...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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