From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [RFC PATCH] discarding swap From: David Woodhouse In-Reply-To: References: <20080910173518.GD20055@kernel.dk> <1221082117.13621.25.camel@macbook.infradead.org> Content-Type: text/plain Date: Fri, 12 Sep 2008 07:09:27 -0700 Message-Id: <1221228567.3919.35.camel@macbook.infradead.org> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Hugh Dickins Cc: Jens Axboe , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org List-ID: On Fri, 2008-09-12 at 13:10 +0100, Hugh Dickins wrote: > So long as the I/O schedulers guarantee that a WRITE bio submitted > to an area already covered by a DISCARD_NOBARRIER bio cannot pass that > DISCARD_NOBARRIER - ... > That seems a reasonable guarantee to me, and perhaps it's trivially > obvious to those who know their I/O schedulers; but I don't, so I'd > like to hear such assurance given. No, that's the point. the I/O schedulers _don't_ give you that guarantee at all. They can treat DISCARD_NOBARRIER just like a write. That's all it is, really -- a special kind of WRITE request without any data. But -- and this came as a bit of a shock to me -- they don't guarantee that writes don't cross writes on their queue. If you issue two WRITE requests to the same sector, you have to make sure for _yourself_ that there is some kind of barrier between them to keep them in the right order. Does swap do that, when a page on the disk is deallocated and then used for something else? -- David Woodhouse Open Source Technology Centre David.Woodhouse@intel.com Intel Corporation -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org