From mboxrd@z Thu Jan 1 00:00:00 1970 From: Volker Lendecke Subject: Re: [PATCH v7 0/5] vfs: Non-blockling buffered fs read (page cache only) Date: Fri, 27 Mar 2015 06:41:25 +0100 Message-ID: References: <20150326202824.65d03787.akpm@linux-foundation.org> Reply-To: Volker.Lendecke@SerNet.DE Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable Return-path: Content-Disposition: inline In-Reply-To: <20150326202824.65d03787.akpm@linux-foundation.org> Sender: owner-linux-aio@kvack.org To: Andrew Morton Cc: Milosz Tanski , linux-kernel@vger.kernel.org, Christoph Hellwig , linux-fsdevel@vger.kernel.org, linux-aio@kvack.org, Mel Gorman , Tejun Heo , Jeff Moyer , Theodore Ts'o , Al Viro , linux-api@vger.kernel.org, Michael Kerrisk , linux-arch@vger.kernel.org, Dave Chinner List-Id: linux-api@vger.kernel.org On Thu, Mar 26, 2015 at 08:28:24PM -0700, Andrew Morton wrote: > A thing which bugs me about pread2() is that it is specifically > tailored to applications which are able to use a partial read result.=20 > ie, by sending it over the network. Can you explain what you mean by this? Samba gets a pread request from a client for some bytes. The client will be confused when we send less than requested although the file is long enough to satisfy all. > And of course fincore could be used by Samba etc to avoid blocking on > reads. It wouldn't perform quite as well as pread2(), but I bet it's > good enough. >=20 > Bottom line: with pread2() there's still a need for fincore(), but with > fincore() there probably isn't a need for pread2(). fincore would be a second syscall per pread, and it is not atomic. I've had discussions with MIPS based vendors who are worried about every single syscall. This is the #1 hottest code path in Samba. > And I'm doubtful about claims that it absolutely has to be non-blocking > 100% of the time. I bet that 99.99% is good enough. A fincore() > option to run mark_page_accessed() against present pages would help > with the race-with-reclaim situation. If you can make sure that after an fincore the pages remain in memory for x milliseconds the atomicity concern might go away. Volker --=20 SerNet GmbH, Bahnhofsallee 1b, 37081 G=F6ttingen phone: +49-551-370000-0, fax: +49-551-370000-9 AG G=F6ttingen, HRB 2816, GF: Dr. Johannes Loxen http://www.sernet.de, mailto:kontakt@sernet.de -- To unsubscribe, send a message with 'unsubscribe linux-aio' in the body to majordomo@kvack.org. For more info on Linux AIO, see: http://www.kvack.org/aio/ Don't email: aart@kvack.org