From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 16 Nov 2015 13:09:50 -0700 From: Ross Zwisler Subject: Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Message-ID: <20151116200950.GB9737@linux.intel.com> References: <1447459610-14259-1-git-send-email-ross.zwisler@linux.intel.com> <1447459610-14259-4-git-send-email-ross.zwisler@linux.intel.com> <22E0F870-C1FB-431E-BF6C-B395A09A2B0D@dilger.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org To: Dan Williams Cc: Andreas Dilger , Ross Zwisler , "linux-kernel@vger.kernel.org" , "H. Peter Anvin" , "J. Bruce Fields" , Theodore Ts'o , Alexander Viro , Dave Chinner , Ingo Molnar , Jan Kara , Jeff Layton , Matthew Wilcox , Thomas Gleixner , linux-ext4 , linux-fsdevel , Linux MM , "linux-nvdimm@lists.01.org" , X86 ML , XFS Developers , Andrew Morton , Matthew Wilcox , Dave Hansen List-ID: On Fri, Nov 13, 2015 at 06:32:40PM -0800, Dan Williams wrote: > On Fri, Nov 13, 2015 at 4:43 PM, Andreas Dilger wrote: > > On Nov 13, 2015, at 5:20 PM, Dan Williams wrote: > >> > >> On Fri, Nov 13, 2015 at 4:06 PM, Ross Zwisler > >> wrote: > >>> Currently the PMEM driver doesn't accept REQ_FLUSH or REQ_FUA bios. These > >>> are sent down via blkdev_issue_flush() in response to a fsync() or msync() > >>> and are used by filesystems to order their metadata, among other things. > >>> > >>> When we get an msync() or fsync() it is the responsibility of the DAX code > >>> to flush all dirty pages to media. The PMEM driver then just has issue a > >>> wmb_pmem() in response to the REQ_FLUSH to ensure that before we return all > >>> the flushed data has been durably stored on the media. > >>> > >>> Signed-off-by: Ross Zwisler > >> > >> Hmm, I'm not seeing why we need this patch. If the actual flushing of > >> the cache is done by the core why does the driver need support > >> REQ_FLUSH? Especially since it's just a couple instructions. REQ_FUA > >> only makes sense if individual writes can bypass the "drive" cache, > >> but no I/O submitted to the driver proper is ever cached we always > >> flush it through to media. > > > > If the upper level filesystem gets an error when submitting a flush > > request, then it assumes the underlying hardware is broken and cannot > > be as aggressive in IO submission, but instead has to wait for in-flight > > IO to complete. > > Upper level filesystems won't get errors when the driver does not > support flush. Those requests are ended cleanly in > generic_make_request_checks(). Yes, the fs still needs to wait for > outstanding I/O to complete but in the case of pmem all I/O is > synchronous. There's never anything to await when flushing at the > pmem driver level. > > > Since FUA/FLUSH is basically a no-op for pmem devices, > > it doesn't make sense _not_ to support this functionality. > > Seems to be a nop either way. Given that DAX may lead to dirty data > pending to the device in the cpu cache that a REQ_FLUSH request will > not touch, its better to leave it all to the mm core to handle. I.e. > it doesn't make sense to call the driver just for two instructions > (sfence + pcommit) when the mm core is taking on the cache flushing. > Either handle it all in the mm or the driver, not a mixture. Does anyone know if ext4 and/or XFS alter their algorithms based on whether the driver supports REQ_FLUSH/REQ_FUA? Will the filesystem behave more efficiently with respect to their internal I/O ordering, etc., if PMEM advertises REQ_FLUSH/REQ_FUA support, even though we could do the same thing at the DAX layer? -- 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 From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752478AbbKPUJ4 (ORCPT ); Mon, 16 Nov 2015 15:09:56 -0500 Received: from mga03.intel.com ([134.134.136.65]:26618 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751498AbbKPUJx (ORCPT ); Mon, 16 Nov 2015 15:09:53 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.20,304,1444719600"; d="scan'208";a="852110538" Date: Mon, 16 Nov 2015 13:09:50 -0700 From: Ross Zwisler To: Dan Williams Cc: Andreas Dilger , Ross Zwisler , "linux-kernel@vger.kernel.org" , "H. Peter Anvin" , "J. Bruce Fields" , "Theodore Ts'o" , Alexander Viro , Dave Chinner , Ingo Molnar , Jan Kara , Jeff Layton , Matthew Wilcox , Thomas Gleixner , linux-ext4 , linux-fsdevel , Linux MM , "linux-nvdimm@lists.01.org" , X86 ML , XFS Developers , Andrew Morton , Matthew Wilcox , Dave Hansen Subject: Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Message-ID: <20151116200950.GB9737@linux.intel.com> Mail-Followup-To: Ross Zwisler , Dan Williams , Andreas Dilger , "linux-kernel@vger.kernel.org" , "H. Peter Anvin" , "J. Bruce Fields" , Theodore Ts'o , Alexander Viro , Dave Chinner , Ingo Molnar , Jan Kara , Jeff Layton , Matthew Wilcox , Thomas Gleixner , linux-ext4 , linux-fsdevel , Linux MM , "linux-nvdimm@lists.01.org" , X86 ML , XFS Developers , Andrew Morton , Matthew Wilcox , Dave Hansen References: <1447459610-14259-1-git-send-email-ross.zwisler@linux.intel.com> <1447459610-14259-4-git-send-email-ross.zwisler@linux.intel.com> <22E0F870-C1FB-431E-BF6C-B395A09A2B0D@dilger.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 13, 2015 at 06:32:40PM -0800, Dan Williams wrote: > On Fri, Nov 13, 2015 at 4:43 PM, Andreas Dilger wrote: > > On Nov 13, 2015, at 5:20 PM, Dan Williams wrote: > >> > >> On Fri, Nov 13, 2015 at 4:06 PM, Ross Zwisler > >> wrote: > >>> Currently the PMEM driver doesn't accept REQ_FLUSH or REQ_FUA bios. These > >>> are sent down via blkdev_issue_flush() in response to a fsync() or msync() > >>> and are used by filesystems to order their metadata, among other things. > >>> > >>> When we get an msync() or fsync() it is the responsibility of the DAX code > >>> to flush all dirty pages to media. The PMEM driver then just has issue a > >>> wmb_pmem() in response to the REQ_FLUSH to ensure that before we return all > >>> the flushed data has been durably stored on the media. > >>> > >>> Signed-off-by: Ross Zwisler > >> > >> Hmm, I'm not seeing why we need this patch. If the actual flushing of > >> the cache is done by the core why does the driver need support > >> REQ_FLUSH? Especially since it's just a couple instructions. REQ_FUA > >> only makes sense if individual writes can bypass the "drive" cache, > >> but no I/O submitted to the driver proper is ever cached we always > >> flush it through to media. > > > > If the upper level filesystem gets an error when submitting a flush > > request, then it assumes the underlying hardware is broken and cannot > > be as aggressive in IO submission, but instead has to wait for in-flight > > IO to complete. > > Upper level filesystems won't get errors when the driver does not > support flush. Those requests are ended cleanly in > generic_make_request_checks(). Yes, the fs still needs to wait for > outstanding I/O to complete but in the case of pmem all I/O is > synchronous. There's never anything to await when flushing at the > pmem driver level. > > > Since FUA/FLUSH is basically a no-op for pmem devices, > > it doesn't make sense _not_ to support this functionality. > > Seems to be a nop either way. Given that DAX may lead to dirty data > pending to the device in the cpu cache that a REQ_FLUSH request will > not touch, its better to leave it all to the mm core to handle. I.e. > it doesn't make sense to call the driver just for two instructions > (sfence + pcommit) when the mm core is taking on the cache flushing. > Either handle it all in the mm or the driver, not a mixture. Does anyone know if ext4 and/or XFS alter their algorithms based on whether the driver supports REQ_FLUSH/REQ_FUA? Will the filesystem behave more efficiently with respect to their internal I/O ordering, etc., if PMEM advertises REQ_FLUSH/REQ_FUA support, even though we could do the same thing at the DAX layer? From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from relay.sgi.com (relay3.corp.sgi.com [198.149.34.15]) by oss.sgi.com (Postfix) with ESMTP id 374977CBF for ; Mon, 16 Nov 2015 14:10:17 -0600 (CST) Received: from cuda.sgi.com (cuda1.sgi.com [192.48.157.11]) by relay3.corp.sgi.com (Postfix) with ESMTP id C966CAC004 for ; Mon, 16 Nov 2015 12:10:13 -0800 (PST) Received: from mga09.intel.com (mga09.intel.com [134.134.136.24]) by cuda.sgi.com with ESMTP id X9afSa9aC48SIS2X for ; Mon, 16 Nov 2015 12:10:09 -0800 (PST) Date: Mon, 16 Nov 2015 13:09:50 -0700 From: Ross Zwisler Subject: Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling Message-ID: <20151116200950.GB9737@linux.intel.com> References: <1447459610-14259-1-git-send-email-ross.zwisler@linux.intel.com> <1447459610-14259-4-git-send-email-ross.zwisler@linux.intel.com> <22E0F870-C1FB-431E-BF6C-B395A09A2B0D@dilger.ca> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: List-Id: XFS Filesystem from SGI List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: xfs-bounces@oss.sgi.com Sender: xfs-bounces@oss.sgi.com To: Dan Williams Cc: Dave Hansen , "J. Bruce Fields" , Linux MM , "H. Peter Anvin" , Jeff Layton , "linux-nvdimm@lists.01.org" , X86 ML , Ingo Molnar , Matthew Wilcox , Ross Zwisler , linux-ext4 , XFS Developers , Alexander Viro , Thomas Gleixner , Andreas Dilger , Theodore Ts'o , "linux-kernel@vger.kernel.org" , Jan Kara , linux-fsdevel , Andrew Morton , Matthew Wilcox On Fri, Nov 13, 2015 at 06:32:40PM -0800, Dan Williams wrote: > On Fri, Nov 13, 2015 at 4:43 PM, Andreas Dilger wrote: > > On Nov 13, 2015, at 5:20 PM, Dan Williams wrote: > >> > >> On Fri, Nov 13, 2015 at 4:06 PM, Ross Zwisler > >> wrote: > >>> Currently the PMEM driver doesn't accept REQ_FLUSH or REQ_FUA bios. These > >>> are sent down via blkdev_issue_flush() in response to a fsync() or msync() > >>> and are used by filesystems to order their metadata, among other things. > >>> > >>> When we get an msync() or fsync() it is the responsibility of the DAX code > >>> to flush all dirty pages to media. The PMEM driver then just has issue a > >>> wmb_pmem() in response to the REQ_FLUSH to ensure that before we return all > >>> the flushed data has been durably stored on the media. > >>> > >>> Signed-off-by: Ross Zwisler > >> > >> Hmm, I'm not seeing why we need this patch. If the actual flushing of > >> the cache is done by the core why does the driver need support > >> REQ_FLUSH? Especially since it's just a couple instructions. REQ_FUA > >> only makes sense if individual writes can bypass the "drive" cache, > >> but no I/O submitted to the driver proper is ever cached we always > >> flush it through to media. > > > > If the upper level filesystem gets an error when submitting a flush > > request, then it assumes the underlying hardware is broken and cannot > > be as aggressive in IO submission, but instead has to wait for in-flight > > IO to complete. > > Upper level filesystems won't get errors when the driver does not > support flush. Those requests are ended cleanly in > generic_make_request_checks(). Yes, the fs still needs to wait for > outstanding I/O to complete but in the case of pmem all I/O is > synchronous. There's never anything to await when flushing at the > pmem driver level. > > > Since FUA/FLUSH is basically a no-op for pmem devices, > > it doesn't make sense _not_ to support this functionality. > > Seems to be a nop either way. Given that DAX may lead to dirty data > pending to the device in the cpu cache that a REQ_FLUSH request will > not touch, its better to leave it all to the mm core to handle. I.e. > it doesn't make sense to call the driver just for two instructions > (sfence + pcommit) when the mm core is taking on the cache flushing. > Either handle it all in the mm or the driver, not a mixture. Does anyone know if ext4 and/or XFS alter their algorithms based on whether the driver supports REQ_FLUSH/REQ_FUA? Will the filesystem behave more efficiently with respect to their internal I/O ordering, etc., if PMEM advertises REQ_FLUSH/REQ_FUA support, even though we could do the same thing at the DAX layer? _______________________________________________ xfs mailing list xfs@oss.sgi.com http://oss.sgi.com/mailman/listinfo/xfs