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Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <20210812122104.GB18532@lst.de> References: <20210812122104.GB18532@lst.de> <162876946134.3068428.15475611190876694695.stgit@warthog.procyon.org.uk> <162876947840.3068428.12591293664586646085.stgit@warthog.procyon.org.uk> To: Christoph Hellwig Cc: dhowells@redhat.com, willy@infradead.org, trond.myklebust@primarydata.com, darrick.wong@oracle.com, jlayton@kernel.org, sfrench@samba.org, torvalds@linux-foundation.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] mm: Make swap_readpage() for SWP_FS_OPS use ->direct_IO() not ->readpage() MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <3085431.1628773025.1@warthog.procyon.org.uk> Date: Thu, 12 Aug 2021 13:57:05 +0100 Message-ID: <3085432.1628773025@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 9B71BD00D1CE X-Stat-Signature: gckpd4cpndoodntc9gacoai3fcey6dat Authentication-Results: imf15.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=ZQZogHzw; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf15.hostedemail.com: domain of dhowells@redhat.com has no SPF policy when checking 216.205.24.124) smtp.mailfrom=dhowells@redhat.com X-HE-Tag: 1628773032-347099 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: Christoph Hellwig wrote: > On Thu, Aug 12, 2021 at 12:57:58PM +0100, David Howells wrote: > > Make swap_readpage(), when accessing a swap file (SWP_FS_OPS) use > > the ->direct_IO() method on the filesystem rather then ->readpage(). > > ->direct_IO is just a helper for ->read_iter and ->write_iter, so please > don't call it directly. It actually is slowly on its way out, with at > at least all of the iomap implementations not using it, as well as various > other file systems. [Note that __swap_writepage() uses ->direct_IO().] Calling ->write_iter is probably a bad idea here. Imagine that it goes through, say, generic_file_write_iter(), then __generic_file_write_iter() and then generic_file_direct_write(). It adds a number of delays into the system, including: - Taking the inode lock - Removing file privs - Cranking mtime, ctime, file version - Doing mnt_want_write - Setting the inode dirty - Waiting on pages in the range that are being written - Walking over the pagecache to invalidate the range - Redoing the invalidation (can't be skipped since page 0 is pinned) that we might want to skip as they'll end up being done for every page swapped out. > > + ki = kzalloc(sizeof(*ki), GFP_KERNEL); > > + if (!ki) > > + return -ENOMEM; > > for the synchronous case we could avoid this allocation and just use > arguments on stack. True. David