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[68.20.15.154]) by smtp.gmail.com with ESMTPSA id s203sm8190618qke.21.2021.10.19.10.13.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 Oct 2021 10:13:05 -0700 (PDT) Message-ID: Subject: Re: [PATCH 01/67] mm: Stop filemap_read() from grabbing a superfluous page From: Jeff Layton To: David Howells , linux-cachefs@redhat.com Cc: Kent Overstreet , "Matthew Wilcox (Oracle)" , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Trond Myklebust , Anna Schumaker , Steve French , Dominique Martinet , Alexander Viro , Omar Sandoval , Linus Torvalds , linux-afs@lists.infradead.org, linux-nfs@vger.kernel.org, linux-cifs@vger.kernel.org, ceph-devel@vger.kernel.org, v9fs-developer@lists.sourceforge.net, linux-kernel@vger.kernel.org Date: Tue, 19 Oct 2021 13:13:04 -0400 In-Reply-To: <163456863216.2614702.6384850026368833133.stgit@warthog.procyon.org.uk> References: <163456861570.2614702.14754548462706508617.stgit@warthog.procyon.org.uk> <163456863216.2614702.6384850026368833133.stgit@warthog.procyon.org.uk> Content-Type: text/plain; charset="ISO-8859-15" User-Agent: Evolution 3.40.4 (3.40.4-2.fc34) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, 2021-10-18 at 15:50 +0100, David Howells wrote: > Under some circumstances, filemap_read() will allocate sufficient pages to > read to the end of the file, call readahead/readpages on them and copy the > data over - and then it will allocate another page at the EOF and call > readpage on that and then ignore it. This is unnecessary and a waste of > time and resources. > > filemap_read() *does* check for this, but only after it has already done > the allocation and I/O. Fix this by checking before calling > filemap_get_pages() also. > > Signed-off-by: David Howells > Acked-by: Kent Overstreet > cc: Matthew Wilcox (Oracle) > cc: linux-mm@kvack.org > cc: linux-fsdevel@vger.kernel.org > Link: https://lore.kernel.org/r/160588481358.3465195.16552616179674485179.stgit@warthog.procyon.org.uk/ > --- > > mm/filemap.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/mm/filemap.c b/mm/filemap.c > index dae481293b5d..c0cdc44c844e 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -2625,6 +2625,10 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter, > if ((iocb->ki_flags & IOCB_WAITQ) && already_read) > iocb->ki_flags |= IOCB_NOWAIT; > > + isize = i_size_read(inode); > + if (unlikely(iocb->ki_pos >= isize)) > + goto put_pages; > + > error = filemap_get_pages(iocb, iter, &pvec); > if (error < 0) > break; > > I would wager that it's worth checking for this. I imagine read calls beyond EOF are common enough that it's probably helpful to optimize that case: Acked-by: Jeff Layton