From: Dan Williams Subject: Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io Date: Mon, 2 May 2016 09:01:58 -0700 Message-ID: References: <1461878218-3844-1-git-send-email-vishal.l.verma@intel.com> <1461878218-3844-6-git-send-email-vishal.l.verma@intel.com> <5727753F.6090104@plexistor.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Cc: Vishal Verma , "linux-nvdimm@lists.01.org" , linux-block@vger.kernel.org, Jan Kara , Matthew Wilcox , Dave Chinner , "linux-kernel@vger.kernel.org" , XFS Developers , Jens Axboe , Linux MM , Al Viro , Christoph Hellwig , linux-fsdevel , Andrew Morton , linux-ext4 To: Boaz Harrosh Return-path: In-Reply-To: <5727753F.6090104@plexistor.com> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Mon, May 2, 2016 at 8:41 AM, Boaz Harrosh wrote: > On 04/29/2016 12:16 AM, Vishal Verma wrote: >> All IO in a dax filesystem used to go through dax_do_io, which cannot >> handle media errors, and thus cannot provide a recovery path that can >> send a write through the driver to clear errors. >> >> Add a new iocb flag for DAX, and set it only for DAX mounts. In the IO >> path for DAX filesystems, use the same direct_IO path for both DAX and >> direct_io iocbs, but use the flags to identify when we are in O_DIRECT >> mode vs non O_DIRECT with DAX, and for O_DIRECT, use the conventional >> direct_IO path instead of DAX. >> > > Really? What are your thinking here? > > What about all the current users of O_DIRECT, you have just made them > 4 times slower and "less concurrent*" then "buffred io" users. Since > direct_IO path will queue an IO request and all. > (And if it is not so slow then why do we need dax_do_io at all? [Rhetorical]) > > I hate it that you overload the semantics of a known and expected > O_DIRECT flag, for special pmem quirks. This is an incompatible > and unrelated overload of the semantics of O_DIRECT. I think it is the opposite situation, it us undoing the premature overloading of O_DIRECT that went in without performance numbers. This implementation clarifies that dax_do_io() handles the lack of a page cache for buffered I/O and O_DIRECT behaves as it nominally would by sending an I/O to the driver. It has the benefit of matching the error semantics of a typical block device where a buffered write could hit an error filling the page cache, but an O_DIRECT write potentially triggers the drive to remap the block.