From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:57660 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726587AbfBYKDl (ORCPT ); Mon, 25 Feb 2019 05:03:41 -0500 Date: Mon, 25 Feb 2019 18:03:26 +0800 From: Ming Lei Subject: Re: [PATCH] xfs: allocate sector sized IO buffer via page_frag_alloc Message-ID: <20190225100325.GA10093@ming.t460p> References: <20190225040904.5557-1-ming.lei@redhat.com> <20190225043648.GE23020@dastard> <20190225084623.GA8397@ming.t460p> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190225084623.GA8397@ming.t460p> Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: Dave Chinner Cc: "Darrick J . Wong" , linux-xfs@vger.kernel.org, Jens Axboe , Vitaly Kuznetsov , Dave Chinner , Christoph Hellwig , Alexander Duyck , Christopher Lameter , Linux FS Devel , linux-mm@kvack.org, linux-block@vger.kernel.org On Mon, Feb 25, 2019 at 04:46:25PM +0800, Ming Lei wrote: > On Mon, Feb 25, 2019 at 03:36:48PM +1100, 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 > > I remembered there is this kind of issue mentioned, but just not found > the details, so post out the patch for restarting the discussion. > > > reused and that pages allocated to the frag pool are pinned in > > memory until all fragments allocated on the page have been freed? > > Yes, that is one problem. But if one page is consumed, sooner or later, > all fragments will be freed, then the page becomes available again. > > > > > i.e. when we consider 64k page machines and 4k block sizes (i.e. > > default config), every single metadata allocation is a sub-page > > allocation and so will use this new page frag mechanism. IOWs, it > > will result in fragmenting memory severely and typical memory > > reclaim not being able to fix it because the metadata that pins each > > page is largely unreclaimable... > > It can be an issue in case of IO timeout & retry. The worst case is still not worse than allocating single page for sub-page IO, which should be used on other file systems under the same situation, I guess. thanks, Ming