Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932074Ab0LATXl (ORCPT ); Wed, 1 Dec 2010 14:23:41 -0500 Received: from smtp-out.google.com ([216.239.44.51]:42810 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756538Ab0LATXi (ORCPT ); Wed, 1 Dec 2010 14:23:38 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=google.com; s=beta; h=date:from:x-x-sender:to:cc:subject:in-reply-to:message-id :references:user-agent:mime-version:content-type; b=A/GhoC/lkniscv9QEx94P1tA2aS99gzzoQe1A4eTu953roJWScbuoOfSILpuS+/jM4 GpMWhPQWtcP/CwB6JnxA== Date: Wed, 1 Dec 2010 11:23:26 -0800 (PST) From: Hugh Dickins X-X-Sender: hughd@tigran.mtv.corp.google.com To: Trond Myklebust cc: Linus Torvalds , Nick Bowler , Linux Kernel Mailing List , linux-nfs@vger.kernel.org, Andrew Morton , Rik van Riel , Christoph Hellwig , Al Viro , Nick Piggin Subject: Re: [PATCH v2 3/3] NFS: Fix a memory leak in nfs_readdir In-Reply-To: <1291229669.6609.24.camel@heimdal.trondhjem.org> Message-ID: References: <1291217804-11257-1-git-send-email-Trond.Myklebust@netapp.com> <1291217804-11257-2-git-send-email-Trond.Myklebust@netapp.com> <20101201150428.GA2879@elliptictech.com> <1291217804-11257-3-git-send-email-Trond.Myklebust@netapp.com> <1291217804-11257-4-git-send-email-Trond.Myklebust@netapp.com> <1291229669.6609.24.camel@heimdal.trondhjem.org> User-Agent: Alpine 2.00 (LSU 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5524 Lines: 124 On Wed, 1 Dec 2010, Trond Myklebust wrote: > On Wed, 2010-12-01 at 08:17 -0800, Linus Torvalds wrote: > > On Wed, Dec 1, 2010 at 7:36 AM, Trond Myklebust > > wrote: > > > > > > We need to ensure that the entries in the nfs_cache_array get cleared > > > when the page is removed from the page cache. To do so, we use the > > > releasepage address_space operation (which also requires us to set > > > the Pg_private flag). > > > > So I really think that the whole "releasepage" use in NFS is simply > > overly complicated and was obviously too subtle. > > > > The whole need for odd return values, for the page lock, and for the > > addition of clearing the up-to-date bit comes from the fact that this > > wasn't really what releasepage was designed for. > > > > 'releasepage' was really designed for the filesystem having its own > > version of 'try_to_free_buffers()', which is just an optimistic "ok, > > we may be releasing this page, so try to get rid of any IO structures > > you have cached". It wasn't really a memory management thing. > > > > And the thing is, it looks trivial to do the memory management > > approach by adding a new callback that gets called after the page is > > actually removed from the page cache. If we do that, then there are no > > races with any other users, since we remove things from the page cache > > atomically wrt page cache lookup. So the need for playing games with > > page locking and 'uptodate' simply goes away. As does the PG_private > > thing or the interaction with invalidatepage() etc. > > > > So this is a TOTALLY UNTESTED trivial patch that just adds another > > callback. Does this work? I dunno. But I get the feeling that instead > > of having NFS work around the odd semantics that don't actually match > > what NFS wants, introducing a new callback with much simpler semantics > > would be simpler for everybody, and avoid the need for subtle code. > > > > Hmm? > > > > Linus > > > > include/linux/fs.h | 1 + > > mm/vmscan.c | 3 +++ > > 2 files changed, 4 insertions(+), 0 deletions(-) > > > > diff --git a/include/linux/fs.h b/include/linux/fs.h > > index c9e06cc..090f0ea 100644 > > --- a/include/linux/fs.h > > +++ b/include/linux/fs.h > > @@ -602,6 +602,7 @@ struct address_space_operations { > > sector_t (*bmap)(struct address_space *, sector_t); > > void (*invalidatepage) (struct page *, unsigned long); > > int (*releasepage) (struct page *, gfp_t); > > + void (*freepage)(struct page *); > > ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec > > *iov, > > loff_t offset, unsigned long nr_segs); > > int (*get_xip_mem)(struct address_space *, pgoff_t, int, > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > index d31d7ce..1accb01 100644 > > --- a/mm/vmscan.c > > +++ b/mm/vmscan.c > > @@ -499,6 +499,9 @@ static int __remove_mapping(struct address_space > > *mapping, struct page *page) > > mem_cgroup_uncharge_cache_page(page); > > } > > > > + if (mapping->a_ops->freepage) > > + mapping->a_ops->freepage(page); > > Hmm... Looking again at the problem, it appears that the same callback > needs to be added to truncate_complete_page() and > invalidate_complete_page2(). Otherwise we end up in a situation where > the page can sometimes be removed from the page cache without calling > freepage(). > > > + > > return 1; > > > > cannot_free: Yes, I was wondering quite how we would define this ->freepage thing, if it gets called from one place that removes from page cache and not from others. Another minor problem with it: it would probably need to take the struct address_space *mapping as arg as well as struct page *page: because by this time page->mapping has been reset to NULL. But I'm not at all keen on adding a calllback in this very special frozen-to-0-references place: please let's not do it without an okay from Nick Piggin (now Cc'ed). I agree completely with what Linus said originally about how the page cannot be freed while there's a reference to it, and it should be possible to work this without your additional page locks. Your ->releasepage should be able to judge whether the page is likely (not certain) to be freed - page_count 3? 1 for the page cache, 1 for the page_private reference, 1 for vmscan's reference, I think. Then it can mark !PageUptodate and proceed with freeing the stuff you had allocated, undo page_has_private and its reference, and return 1 (or return 0 if it decides to hold on to the page). If something races in and grabs another reference to prevent removal from page cache and freeing, then won't read_cache_page(), seeing !Uptodate, do the right thing and set up the required info again? Or perhaps I haven't looked far enough, and you do have races which actually need your page locks, I can see they make it easier to think about. But I'd prefer us not to throw in another callback if it's well workable with the ->releasepage we already have. (If it helps, perhaps we could adjust shrink_page_list() to allow for the page being removed from page cache inside try_to_release_page() - but I don't think that should be necessary.) Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/