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Received: from viro by zeniv.linux.org.uk with local (Exim 4.96 #2 (Red Hat Linux)) id 1ppctd-00Az06-1Y; Thu, 20 Apr 2023 22:36:57 +0000 Date: Thu, 20 Apr 2023 23:36:57 +0100 From: Al Viro To: David Howells Cc: Jens Axboe , Christoph Hellwig , Matthew Wilcox , Jan Kara , Jeff Layton , David Hildenbrand , Jason Gunthorpe , Logan Gunthorpe , Hillf Danton , Christian Brauner , Linus Torvalds , linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Christoph Hellwig , John Hubbard Subject: Re: [PATCH v18 09/15] iov_iter: Kill ITER_PIPE Message-ID: <20230420223657.GV3390869@ZenIV> References: <20230314220757.3827941-1-dhowells@redhat.com> <20230314220757.3827941-10-dhowells@redhat.com> <20230420222231.GT3390869@ZenIV> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230420222231.GT3390869@ZenIV> X-Stat-Signature: 5a1f1ggo3jd6uwnndz771wronwskzmaj X-Rspam-User: X-Rspamd-Queue-Id: 251FD120012 X-Rspamd-Server: rspam06 X-HE-Tag: 1682030233-108520 X-HE-Meta: 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 jycHMb2z Iya06tg9awJoDDDF81IcpLggab9UDbex0MpKmg910Q8BB3TdQfoXkLZuPpANu7e59fkZv6UK75ahSJYHHUTC2g5J6NFMOxDm/Ua2OcdKpPSDW9HVUBrmfnWMBVn0OaYLS9m0NnkC8MJxEMDuJUsNHLBtKBB2nRQI/fWD/HxlntQJfQUdRdNuqWl7G1i56wtxiN3LUKUxsdvpQsykEEdS5XHby1rS/OvowOwY9DkquO3GOXaVY5od3ui88DVhweiZcwbx4b+RyxJzvL9SgXsZHiUey8GBEkmJu8jkRJqxvaTSpz/kUWG1WZEgTfw/QTLKrz+bpLabSgBfUOkpdJD+HjbIc3JjOrg+1udKPNvLX1L7BWay5uYWiW0wHqtPVza6T40izWWtwxJKj99QZIwiM7YuinA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Apr 20, 2023 at 11:22:31PM +0100, Al Viro wrote: > On Tue, Mar 14, 2023 at 10:07:51PM +0000, David Howells wrote: > > The ITER_PIPE-type iterator was only used for generic_file_splice_read(), > > but that has now been switched to either pull pages directly from the > > pagecache for buffered file splice-reads or to use ITER_BVEC instead for > > O_DIRECT file splice-reads. This leaves ITER_PIPE unused - so remove it. > > Wonderful, except that now you've got duplicates of ->read_iter() for > everyone who wants zero-copy on ->splice_read() ;-/ > > I understand the attraction of arbitrary seeks on those suckers; ITER_PIPE > is a massive headache in that respect. But I really don't like what your > approach trades it for. > > And you are nowhere near done - consider e.g. NFS. Mainline has it > feed ITER_PIPE to nfs_file_read(), which does call generic_file_read_iter() - > after > result = nfs_revalidate_mapping(inode, iocb->ki_filp->f_mapping); > > Sure, you can add nfs_file_splice_read() that would do what nfs_file_read() > does, calling filemap_spice_read() instead of generic_file_read_iter(). > > Repeat the same for ocfs2 (locking of its own). And orangefs. And > XFS (locking, again). And your own AFS, while we are at it. Et sodding > cetera - *everything* that uses generic_file_splice_read() with > ->read_iter other than generic_file_read_iter() needs review and, > quite likely, a ->splice_read() instance of its own. Don't get me wrong - I'd love to kill ITER_PIPE off; it's just that tons of ->splice_read() duplicating the corresponding ->read_iter() up to the point where it would call generic_file_read_iter(), modulo ignoring O_DIRECT case and then calling filemap_splice_read() instead... Painful and asking for trouble down the road.