From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f198.google.com (mail-io0-f198.google.com [209.85.223.198]) by kanga.kvack.org (Postfix) with ESMTP id EE7846B007E for ; Mon, 25 Apr 2016 13:21:45 -0400 (EDT) Received: by mail-io0-f198.google.com with SMTP id k129so100203157iof.0 for ; Mon, 25 Apr 2016 10:21:45 -0700 (PDT) Received: from mail-oi0-x22a.google.com (mail-oi0-x22a.google.com. [2607:f8b0:4003:c06::22a]) by mx.google.com with ESMTPS id w6si2974043otb.155.2016.04.25.10.21.45 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 25 Apr 2016 10:21:45 -0700 (PDT) Received: by mail-oi0-x22a.google.com with SMTP id r78so183828942oie.0 for ; Mon, 25 Apr 2016 10:21:45 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <1461604476.3106.12.camel@intel.com> References: <1459303190-20072-1-git-send-email-vishal.l.verma@intel.com> <1459303190-20072-6-git-send-email-vishal.l.verma@intel.com> <20160420205923.GA24797@infradead.org> <1461434916.3695.7.camel@intel.com> <20160425083114.GA27556@infradead.org> <1461604476.3106.12.camel@intel.com> Date: Mon, 25 Apr 2016 10:21:44 -0700 Message-ID: Subject: Re: [PATCH v2 5/5] dax: handle media errors in dax_do_io From: Dan Williams Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: "Verma, Vishal L" Cc: "hch@infradead.org" , "axboe@fb.com" , "jack@suse.cz" , "linux-nvdimm@ml01.01.org" , "david@fromorbit.com" , "linux-kernel@vger.kernel.org" , "xfs@oss.sgi.com" , "linux-block@vger.kernel.org" , "linux-mm@kvack.org" , "Wilcox, Matthew R" , "linux-fsdevel@vger.kernel.org" , "akpm@linux-foundation.org" , "linux-ext4@vger.kernel.org" , "viro@zeniv.linux.org.uk" On Mon, Apr 25, 2016 at 10:14 AM, Verma, Vishal L wrote: > On Mon, 2016-04-25 at 01:31 -0700, hch@infradead.org wrote: >> On Sat, Apr 23, 2016 at 06:08:37PM +0000, Verma, Vishal L wrote: >> > >> > direct_IO might fail with -EINVAL due to misalignment, or -ENOMEM >> > due >> > to some allocation failing, and I thought we should return the >> > original >> > -EIO in such cases so that the application doesn't lose the >> > information >> > that the bad block is actually causing the error. >> EINVAL is a concern here. Not due to the right error reported, but >> because it means your current scheme is fundamentally broken - we >> need to support I/O at any alignment for DAX I/O, and not fail due to >> alignbment concernes for a highly specific degraded case. >> >> I think this whole series need to go back to the drawing board as I >> don't think it can actually rely on using direct I/O as the EIO >> fallback. >> > Agreed that DAX I/O can happen with any size/alignment, but how else do > we send an IO through the driver without alignment restrictions? Also, > the granularity at which we store badblocks is 512B sectors, so it > seems natural that to clear such a sector, you'd expect to send a write > to the whole sector. > > The expected usage flow is: > > - Application hits EIO doing dax_IO or load/store io > > - It checks badblocks and discovers it's files have lost data > > - It write()s those sectors (possibly converted to file offsets using > fiemap) > * This triggers the fallback path, but if the application is doing > this level of recovery, it will know the sector is bad, and write the > entire sector > > - Or it replaces the entire file from backup also using write() (not > mmap+stores) > * This just frees the fs block, and the next time the block is > reallocated by the fs, it will likely be zeroed first, and that will be > done through the driver and will clear errors > > > I think if we want to keep allowing arbitrary alignments for the > dax_do_io path, we'd need: > 1. To represent badblocks at a finer granularity (likely cache lines) > 2. To allow the driver to do IO to a *block device* at sub-sector > granularity 3. Arrange for O_DIRECT to bypass dax_do_io(), and leave the optimization only for the dax "buffered I/O" case. 4. Skip dax_do_io() entirely in the presence of errors I think 3 is the most closely aligned with the typical block device model. In the typical case a buffered write may fail due to a badblock read when filling the page cache, but an O_DIRECT write would bypass the page cache and potentially clear the error / cause the block to be reallocated internally to the drive. -- 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