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Sat, 21 Mar 2020 03:58:33 +1100 (AEDT) Received: from sol.localdomain (c-107-3-166-239.hsd1.ca.comcast.net [107.3.166.239]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 6434C20753; Fri, 20 Mar 2020 16:58:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584723510; bh=AxLDPN4LwuxzW4xjwlSgsVf6rtc0JFVKxbo5//wmPgc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=cRbat8REA7EmmJnmLCrWxRoNW5LgyCKm+4EsNDf5E4lY0nLcYrSE6Jyr+LkCnKmyi n3W2x46nv8ZHmzqeZMc86Mj56kXxAhIEwG1PHXCTucYL5iivPYgJLLRVtDE0DPEV3X iA5d4+HsRZ/BNe0ydHe/SfLeiRxRKGyEVqIcRkok= Date: Fri, 20 Mar 2020 09:58:28 -0700 From: Eric Biggers To: Matthew Wilcox Subject: Re: [PATCH v9 12/25] mm: Move end_index check out of readahead loop Message-ID: <20200320165828.GB851@sol.localdomain> References: <20200320142231.2402-1-willy@infradead.org> <20200320142231.2402-13-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200320142231.2402-13-willy@infradead.org> X-BeenThere: linux-erofs@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Development of Linux EROFS file system List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: cluster-devel@redhat.com, linux-mm@kvack.org, John Hubbard , linux-kernel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-xfs@vger.kernel.org, William Kucharski , linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , linux-ext4@vger.kernel.org, linux-erofs@lists.ozlabs.org, ocfs2-devel@oss.oracle.com Errors-To: linux-erofs-bounces+linux-erofs=archiver.kernel.org@lists.ozlabs.org Sender: "Linux-erofs" On Fri, Mar 20, 2020 at 07:22:18AM -0700, Matthew Wilcox wrote: > From: "Matthew Wilcox (Oracle)" > > By reducing nr_to_read, we can eliminate this check from inside the loop. > > Signed-off-by: Matthew Wilcox (Oracle) > Reviewed-by: John Hubbard > Reviewed-by: William Kucharski > --- > mm/readahead.c | 17 +++++++++++------ > 1 file changed, 11 insertions(+), 6 deletions(-) > > diff --git a/mm/readahead.c b/mm/readahead.c > index d01531ef9f3c..a37b68f66233 100644 > --- a/mm/readahead.c > +++ b/mm/readahead.c > @@ -167,8 +167,6 @@ void __do_page_cache_readahead(struct address_space *mapping, > unsigned long lookahead_size) > { > struct inode *inode = mapping->host; > - struct page *page; > - unsigned long end_index; /* The last page we want to read */ > LIST_HEAD(page_pool); > loff_t isize = i_size_read(inode); > gfp_t gfp_mask = readahead_gfp_mask(mapping); > @@ -178,22 +176,29 @@ void __do_page_cache_readahead(struct address_space *mapping, > ._index = index, > }; > unsigned long i; > + pgoff_t end_index; /* The last page we want to read */ > > if (isize == 0) > return; > > - end_index = ((isize - 1) >> PAGE_SHIFT); > + end_index = (isize - 1) >> PAGE_SHIFT; > + if (index > end_index) > + return; > + /* Avoid wrapping to the beginning of the file */ > + if (index + nr_to_read < index) > + nr_to_read = ULONG_MAX - index + 1; > + /* Don't read past the page containing the last byte of the file */ > + if (index + nr_to_read >= end_index) > + nr_to_read = end_index - index + 1; There seem to be a couple off-by-one errors here. Shouldn't it be: /* Avoid wrapping to the beginning of the file */ if (index + nr_to_read < index) nr_to_read = ULONG_MAX - index; /* Don't read past the page containing the last byte of the file */ if (index + nr_to_read > end_index) nr_to_read = end_index - index + 1; I.e., 'ULONG_MAX - index' rather than 'ULONG_MAX - index + 1', so that 'index + nr_to_read' is then ULONG_MAX rather than overflowed to 0. Then 'index + nr_to_read > end_index' rather 'index + nr_to_read >= end_index', since otherwise nr_to_read can be increased by 1 rather than decreased or stay the same as expected. - Eric