From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Fri, 23 Nov 2018 09:59:32 -0800 From: Matthew Wilcox To: "Kirill A. Shutemov" Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" , Hugh Dickins Subject: Re: [PATCH 1/2] mm: Remove redundant test from find_get_pages_contig Message-ID: <20181123175932.GV3065@bombadil.infradead.org> References: <20181122213224.12793-1-willy@infradead.org> <20181122213224.12793-2-willy@infradead.org> <20181123104732.gvhdqyddbsiq3i42@kshutemo-mobl1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181123104732.gvhdqyddbsiq3i42@kshutemo-mobl1> Sender: owner-linux-mm@kvack.org List-ID: On Fri, Nov 23, 2018 at 01:47:32PM +0300, Kirill A. Shutemov wrote: > On Thu, Nov 22, 2018 at 01:32:23PM -0800, Matthew Wilcox wrote: > > After we establish a reference on the page, we check the pointer continues > > to be in the correct position in i_pages. There's no need to check the > > page->mapping or page->index afterwards; if those can change after we've > > got the reference, they can change after we return the page to the caller. > > Hm. IIRC, page->mapping can be set to NULL due truncation, but what about > index? When it can be changed? Truncation doesn't touch it. I think index can only be changed after the refcount has hit zero and the page is safely out of the pagecache. I agree that page->mapping can be set to NULL after the call to xas_reload() ... but then it can also happen after the check, so the check isn't really buying us anything that the xas_reload() call doesn't already check.