From: Dan Williams Subject: Re: [PATCH v3 2/2] ext4: handle layout changes to pinned DAX mappings Date: Thu, 5 Jul 2018 22:08:33 -0700 Message-ID: References: <20180627212252.31032-3-ross.zwisler@linux.intel.com> <20180702172912.329-1-ross.zwisler@linux.intel.com> <20180704004923.GT19934@dastard> <20180704122723.lup2wovzb6u6ta6v@quack2.suse.cz> <20180704235414.GU19934@dastard> <20180705035952.GB5699@magnolia> <20180705232934.GW19934@dastard> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: ext4 hackers , "Darrick J. Wong" , Jan Kara , Christoph Hellwig , "linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org" To: Dave Chinner Return-path: In-Reply-To: <20180705232934.GW19934@dastard> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-nvdimm-bounces-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org Sender: "Linux-nvdimm" List-Id: linux-ext4.vger.kernel.org On Thu, Jul 5, 2018 at 4:29 PM Dave Chinner wrote: > On Thu, Jul 05, 2018 at 01:40:17PM -0700, Dan Williams wrote: > > On Wed, Jul 4, 2018 at 8:59 PM, Darrick J. Wong > wrote: > > > On Thu, Jul 05, 2018 at 09:54:14AM +1000, Dave Chinner wrote: > > >> On Wed, Jul 04, 2018 at 02:27:23PM +0200, Jan Kara wrote: > > >> > On Wed 04-07-18 10:49:23, Dave Chinner wrote: > > >> > > On Mon, Jul 02, 2018 at 11:29:12AM -0600, Ross Zwisler wrote: > > >> > > > Follow the lead of xfs_break_dax_layouts() and add > synchronization between > > >> > > > operations in ext4 which remove blocks from an inode (hole > punch, truncate > > >> > > > down, etc.) and pages which are pinned due to DAX DMA > operations. > > >> > > > > > >> > > > Signed-off-by: Ross Zwisler > > >> > > > Reviewed-by: Jan Kara > > >> > > > Reviewed-by: Lukas Czerner > > >> > > > --- > > >> > > > > > >> > > > Changes since v2: > > >> > > > * Added a comment to ext4_insert_range() explaining why we > don't call > > >> > > > ext4_break_layouts(). (Jan) > > >> > > > > >> > > Which I think is wrong and will cause data corruption. > > >> > > > > >> > > > @@ -5651,6 +5663,11 @@ int ext4_insert_range(struct inode > *inode, loff_t offset, loff_t len) > > >> > > > LLONG_MAX); > > >> > > > if (ret) > > >> > > > goto out_mmap; > > >> > > > + /* > > >> > > > + * We don't need to call ext4_break_layouts() because > we aren't > > >> > > > + * removing any blocks from the inode. We are just > changing their > > >> > > > + * offset by inserting a hole. > > >> > > > + */ > > > > > > Does calling ext4_break_layouts from insert range not work? > > > > > > It's my understanding that file leases work are a mechanism for the > > > filesystem to delegate some of its authority over physical space > > > mappings to "client" software. AFAICT it's used for mmap'ing pmem > > > directly into userspace and exporting space on shared storage over > > > pNFS. Some day we might use the same mechanism for the similar things > > > that RDMA does, or the swapfile code since that's essentially how it > > > works today. > > > > > > The other part of these file leases is that the filesystem revokes them > > > any time it wants to perform a mapping operation on a file. This > breaks > > > my mental model of how leases work, and if you commit to this for ext4 > > > then I'll have to remember that leases are different between xfs and > > > ext4. Worse, since the reason for skipping ext4_break_layouts seems to > > > be the implementation detail that "DAX won't care", then someone else > > > wiring up pNFS/future RDMA/whatever will also have to remember to put > it > > > back into ext4 or else kaboom. > > > > > > Granted, Dave said all these things already, but I actually feel > > > strongly enough to reiterate. > > > > This patch kit is only for the DAX fix, this isn't full layout lease > > support. Even XFS is special casing unmap with the BREAK_UNMAP flag. > > So ext4 is achieving feature parity for BREAK_UNMAP, just not > > BREAK_WRITE, yet. > > BREAK_UNMAP is issued unconditionally by XFS for all fallocate > operations. There is no special except for extent shifting (up or > down) in XFS as this patch set is making for ext4. IOWs, this > patchset does not implement BREAK_UNMAP with the same semantics as > XFS. Ah true, I see that now.