From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 467A0C4332F for ; Fri, 30 Sep 2022 13:41:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231569AbiI3Nl4 (ORCPT ); Fri, 30 Sep 2022 09:41:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52496 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231547AbiI3Nlv (ORCPT ); Fri, 30 Sep 2022 09:41:51 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [IPv6:2001:67c:2178:6::1d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 58D6717CCC9; Fri, 30 Sep 2022 06:41:46 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id 06FE61F88E; Fri, 30 Sep 2022 13:41:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1664545305; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=W3iEPJPd4WNALUdHRRfTjBi+oXPYskJSYk9FE5ApHVI=; b=c4bGmWjEcN9VYtgaWDbnSVcoO3a87aGSWoLgqHJs7Ullw8llaYGbsBG8aP3wJuZ0WNUK0e dyClrZQb+spR2ajvH31k0mJoGOWbURztSwchybbhwigBOPJeaJnD3ohfqPafwY1HHYZlcW cBj6YiTNYZsXDWSU9/v4GMQc6knQRvg= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1664545305; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=W3iEPJPd4WNALUdHRRfTjBi+oXPYskJSYk9FE5ApHVI=; b=uIKW4D3ioWwIGLwYUWsLLXWvSLtf9wAKT6KY+LoAMLmlCeVKyhptUh66PCD+dxwXR6PIum Y543b8qOh6WAmFBg== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id EB16813776; Fri, 30 Sep 2022 13:41:44 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id e49fORjyNmNrRgAAMHmgww (envelope-from ); Fri, 30 Sep 2022 13:41:44 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 809A0A0668; Fri, 30 Sep 2022 15:41:44 +0200 (CEST) Date: Fri, 30 Sep 2022 15:41:44 +0200 From: Jan Kara To: Dan Williams Cc: Jan Kara , Dave Chinner , Jason Gunthorpe , akpm@linux-foundation.org, Matthew Wilcox , "Darrick J. Wong" , Christoph Hellwig , John Hubbard , linux-fsdevel@vger.kernel.org, nvdimm@lists.linux.dev, linux-xfs@vger.kernel.org, linux-mm@kvack.org, linux-ext4@vger.kernel.org Subject: Re: [PATCH v2 05/18] xfs: Add xfs_break_layouts() to the inode eviction path Message-ID: <20220930134144.pd67rbgahzcb62mf@quack3> References: <6329ee04c9272_2a6ded294bf@dwillia2-xfh.jf.intel.com.notmuch> <20220921221416.GT3600936@dread.disaster.area> <20220923001846.GX3600936@dread.disaster.area> <632d00a491d0d_4a67429488@dwillia2-xfh.jf.intel.com.notmuch> <20220923021012.GZ3600936@dread.disaster.area> <20220923093803.nroajmvn7twuptez@quack3> <20220925235407.GA3600936@dread.disaster.area> <20220926141055.sdlm3hkfepa7azf2@quack3> <63362b4781294_795a6294f0@dwillia2-xfh.jf.intel.com.notmuch> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <63362b4781294_795a6294f0@dwillia2-xfh.jf.intel.com.notmuch> Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Thu 29-09-22 16:33:27, Dan Williams wrote: > Jan Kara wrote: > > On Mon 26-09-22 09:54:07, Dave Chinner wrote: > > > > I'd be more worried about stuff like vmsplice() that can add file pages > > > > into pipe without holding inode alive in any way and keeping them there for > > > > arbitrarily long time. Didn't we want to add FOLL_LONGTERM to gup executed > > > > from vmsplice() to avoid issues like this? > > > > > > Yes, ISTR that was part of the plan - use FOLL_LONGTERM to ensure > > > FSDAX can't run operations that pin pages but don't take fs > > > references. I think that's how we prevented RDMA users from pinning > > > FSDAX direct mapped storage media in this way. It does not, however, > > > prevent the above "short term" GUP UAF situation from occurring. > > > > If what I wrote above is correct, then I understand and agree. > > > > > > I agree that freeing VMA while there are pinned pages is ... inconvenient. > > > > But that is just how gup works since the beginning - the moment you have > > > > struct page reference, you completely forget about the mapping you've used > > > > to get to the page. So anything can happen with the mapping after that > > > > moment. And in case of pages mapped by multiple processes I can easily see > > > > that one of the processes decides to unmap the page (and it may well be > > > > that was the initial process that acquired page references) while others > > > > still keep accessing the page using page references stored in some internal > > > > structure (RDMA anyone?). > > > > > > Yup, and this is why RDMA on FSDAX using this method of pinning pages > > > will end up corrupting data and filesystems, hence FOLL_LONGTERM > > > protecting against most of these situations from even arising. But > > > that's that workaround, not a long term solution that allows RDMA to > > > be run on FSDAX managed storage media. > > > > > > I said on #xfs a few days ago: > > > > > > [23/9/22 10:23] * dchinner is getting deja vu over this latest round > > > of "dax mappings don't pin the filesystem objects that own the > > > storage media being mapped" > > > > > > And I'm getting that feeling again right now... > > > > > > > I think it will be rather difficult to come up > > > > with some scheme keeping VMA alive while there are pages pinned without > > > > regressing userspace which over the years became very much tailored to the > > > > peculiar gup behavior. > > > > > > Perhaps all we should do is add a page flag for fsdax mapped pages > > > that says GUP must pin the VMA, so only mapped pages that fall into > > > this category take the perf penalty of VMA management. > > > > Possibly. But my concern with VMA pinning was not only about performance > > but also about applications relying on being able to unmap pages that are > > currently pinned. At least from some processes one of which may be the one > > doing the original pinning. But yeah, the fact that FOLL_LONGTERM is > > forbidden with DAX somewhat restricts the insanity we have to deal with. So > > maybe pinning the VMA for DAX mappings might actually be a workable > > solution. > > As far as I can see, VMAs are not currently reference counted they are > just added / deleted from an mm_struct, and nothing guarantees > mapping_mapped() stays true while a page is pinned. I agree this solution requires quite some work. But I wanted to say that in principle it would be a logically consistent and technically not that difficult solution. > I like Dave's mental model that the inode is the arbiter for the page, > and the arbiter is not allowed to go out of scope before asserting that > everything it granted previously has been returned. > > write_inode_now() unconditionally invokes dax_writeback_mapping_range() > when the inode is committed to going out of scope. write_inode_now() is > allowed to sleep until all dirty mapping entries are written back. I see > nothing wrong with additionally checking for entries with elevated page > reference counts and doing a: > > __wait_var_event(page, dax_page_idle(page)); > > Since the inode is out of scope there should be no concerns with racing > new 0 -> 1 page->_refcount transitions. Just wait for transient page > pins to finally drain to zero which should already be on the order of > the wait time to complete synchrounous writeback in the dirty inode > case. I agree this is doable but there's the nasty sideeffect that inode reclaim may block for abitrary time waiting for page pinning. If the application that has pinned the page requires __GFP_FS memory allocation to get to a point where it releases the page, we even have a deadlock possibility. So it's better than the UAF issue but still not ideal. Honza -- Jan Kara SUSE Labs, CR