From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: Dmitry Monakhov To: Jan Kara , Ross Zwisler Cc: linux-nvdimm@lists.01.org, Dave Chinner , linux-kernel@vger.kernel.org, linux-mm@kvack.org, mgorman@suse.de, linux-fsdevel@vger.kernel.org Subject: Re: Another proposal for DAX fault locking In-Reply-To: <20160209172416.GB12245@quack.suse.cz> References: <20160209172416.GB12245@quack.suse.cz> Date: Wed, 10 Feb 2016 15:29:34 +0300 Message-ID: <87egck4ukx.fsf@openvz.org> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha512; protocol="application/pgp-signature" Sender: owner-linux-mm@kvack.org List-ID: --=-=-= Content-Type: text/plain Content-Transfer-Encoding: quoted-printable Jan Kara writes: > Hello, > > I was thinking about current issues with DAX fault locking [1] (data > corruption due to racing faults allocating blocks) and also races which > currently don't allow us to clear dirty tags in the radix tree due to rac= es > between faults and cache flushing [2]. Both of these exist because we don= 't > have an equivalent of page lock available for DAX. While we have a > reasonable solution available for problem [1], so far I'm not aware of a > decent solution for [2]. After briefly discussing the issue with Mel he h= ad > a bright idea that we could used hashed locks to deal with [2] (and I thi= nk > we can solve [1] with them as well). So my proposal looks as follows: > > DAX will have an array of mutexes (the array can be made per device but > initially a global one should be OK). We will use mutexes in the array as= a > replacement for page lock - we will use hashfn(mapping, index) to get > particular mutex protecting our offset in the mapping. On fault / page > mkwrite, we'll grab the mutex similarly to page lock and release it once = we > are done updating page tables. This deals with races in [1]. When flushing > caches we grab the mutex before clearing writeable bit in page tables > and clearing dirty bit in the radix tree and drop it after we have flushed > caches for the pfn. This deals with races in [2]. > > Thoughts? Agree, only small note: Hash locks has side effect for batch locking due to collision. Some times we want to lock several pages/entries (migration/defragmentation) So we will endup with deadlock due to hash collision. > > Honza > > [1] http://oss.sgi.com/archives/xfs/2016-01/msg00575.html > [2] https://lists.01.org/pipermail/linux-nvdimm/2016-January/004057.html > > --=20 > Jan Kara > SUSE Labs, CR > _______________________________________________ > Linux-nvdimm mailing list > Linux-nvdimm@lists.01.org > https://lists.01.org/mailman/listinfo/linux-nvdimm --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBCgAGBQJWuy0uAAoJELhyPTmIL6kBG5cIAJXSd1Ta5jXS9UR4eWvbSrvb s1dDJ9/mSsczuXhCVFdsWd6JkweN3xHYaLyqbkUwNdk8qgsjMZCoAiggHK5/zF5k 7RAMhTyuM7n2NyRfYztG+YOI+713X4V5h3sPolY0wCzUMbbze7M9kYBmFrenMj3H qSvkN0lB5CO1C3NsZPmnga5uIBD11ony6ZP8sIRkZdB/M7GKK6n3UHvVAN2ulMJ/ y706UznvEH7VgMExYftjvME5VayoV0ktrKxQJJfxUJZcZC19HW7NC0rGkuny54Qr 0zmxw4TJLvAXolu73hgaShgOjTYKdgrTMC5iwl62v0L4unQAUu3Cc770tIUtH2Q= =of95 -----END PGP SIGNATURE----- --=-=-=-- -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org