From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Christie Subject: Re: allowed pages in the block later, was Re: [Ext2-devel] [PATCH] ext3: avoid sending down non-refcounted pages Date: Thu, 08 Dec 2005 12:22:37 -0600 Message-ID: <439879ED.5050706@cs.wisc.edu> References: <20051208180900T.fujita.tomonori@lab.ntt.co.jp> <20051208101833.GM14509@schatzie.adilger.int> <20051208134239.GA13376@infradead.org> <439878E4.6060505@cs.wisc.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Christoph Hellwig , FUJITA Tomonori , linux-fsdevel@vger.kernel.org, ext2-devel@lists.sourceforge.net, linux-mm@kvack.org, linux-kernel@vger.kernel.org Return-path: Received: from sabe.cs.wisc.edu ([128.105.6.20]:8677 "EHLO sabe.cs.wisc.edu") by vger.kernel.org with ESMTP id S932228AbVLHSW7 (ORCPT ); Thu, 8 Dec 2005 13:22:59 -0500 To: open-iscsi@googlegroups.com In-Reply-To: <439878E4.6060505@cs.wisc.edu> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org Mike Christie wrote: > > Christoph Hellwig wrote: > >> On Thu, Dec 08, 2005 at 03:18:33AM -0700, Andreas Dilger wrote: >> >>> What happens on 1kB or 2kB block filesystems (i.e. b_size != PAGE_SIZE)? >>> This will allocate a whole page for each block (which may be >>> considerable >>> overhead on e.g. a 64kB PAGE_SIZE ia64 or PPC system). >> >> >> >> Yes. How often do we trigger this codepath? >> >> The problem we're trying to solve here is how do implement network block >> devices (nbd, iscsi) efficiently. The zero copy codepath in the >> networking >> layer does need to grab additional references to pages. So to use >> sendpage >> we need a refcountable page. pages used by the slab allocator are not >> normally refcounted so try to do get_page/pub_page on them will break. >> >> One way to work around that would be to detect kmalloced pages and use >> a slowpath for that. The major issues with that is that we don't have a >> reliable way to detect if a given struct page comes from the slab >> allocator >> or not. The minor problem is that even with such an indicator it means >> having a separate and lightly tested slowpath for this rare case. >> >> All in all I think we should document that the block layer only accepts >> properly refcounted pages, which is everything but kmalloced pages (even >> vmalloc is totally fine) > > > Is it anytime kmalloc is used? For scsi when it uses scsi_execute* for > something like scanning (report luns result is kmallocd) would this be a > problem? > > If PageSlab() does work, then could we have a request queue flag that > bounces those pages for all block layer drivers. Pretty slow and yucky > but if we have to convert SCSI and maybe other parts of the block layer > maybe it will be easiest for now. > Or there is not a way to do kmalloc(GFP_BLK) that gives us the right type of memory is there?