From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:45888 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725929AbfBZCXH (ORCPT ); Mon, 25 Feb 2019 21:23:07 -0500 Date: Tue, 26 Feb 2019 10:22:50 +0800 From: Ming Lei Subject: Re: [PATCH] xfs: allocate sector sized IO buffer via page_frag_alloc Message-ID: <20190226022249.GA17747@ming.t460p> References: <20190225040904.5557-1-ming.lei@redhat.com> <20190225043648.GE23020@dastard> <5ad2ef83-8b3a-0a15-d72e-72652b807aad@suse.cz> <20190225202630.GG23020@dastard> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190225202630.GG23020@dastard> Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: Dave Chinner Cc: Vlastimil Babka , "Darrick J . Wong" , linux-xfs@vger.kernel.org, Jens Axboe , Vitaly Kuznetsov , Dave Chinner , Christoph Hellwig , Alexander Duyck , Aaron Lu , Christopher Lameter , Linux FS Devel , linux-mm@kvack.org, linux-block@vger.kernel.org On Tue, Feb 26, 2019 at 07:26:30AM +1100, Dave Chinner wrote: > On Mon, Feb 25, 2019 at 02:15:59PM +0100, Vlastimil Babka wrote: > > On 2/25/19 5:36 AM, Dave Chinner wrote: > > > On Mon, Feb 25, 2019 at 12:09:04PM +0800, Ming Lei wrote: > > >> XFS uses kmalloc() to allocate sector sized IO buffer. > > > .... > > >> Use page_frag_alloc() to allocate the sector sized buffer, then the > > >> above issue can be fixed because offset_in_page of allocated buffer > > >> is always sector aligned. > > > > > > Didn't we already reject this approach because page frags cannot be > > > reused and that pages allocated to the frag pool are pinned in > > > memory until all fragments allocated on the page have been freed? > > > > I don't know if you did, but it's certainly true., Also I don't think > > there's any specified alignment guarantee for page_frag_alloc(). > > We did, and the alignment guarantee would have come from all > fragments having an aligned size. > > > What about kmem_cache_create() with align parameter? That *should* be > > guaranteed regardless of whatever debugging is enabled - if not, I would > > consider it a bug. > > Yup, that's pretty much what was decided. The sticking point was > whether is should be block layer infrastructure (because the actual > memory buffer alignment is a block/device driver requirement not > visible to the filesystem) or whether "sector size alignement is > good enough for everyone". OK, looks I miss the long life time of meta data caching, then let's discuss the slab approach. Looks one single slab cache doesn't work, given the size may be 512 * N (1 <= N < PAGE_SIZE/512), that is basically what I posted the first time. https://marc.info/?t=153986884900007&r=1&w=2 https://marc.info/?t=153986885100001&r=1&w=2 Or what is the exact size of sub-page IO in xfs most of time? For example, if 99% times falls in 512 byte allocation, maybe it is enough to just maintain one 512byte slab. Thanks, Ming