Date: Wed, 4 Jul 2012 12:06:33 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Pavlo <devgs@ukr.net> Cc: freebsd-fs@freebsd.org Subject: Re: mmap() incoherency on hi I/O load (FS is zfs) Message-ID: <20120704090633.GH2337@deviant.kiev.zoral.com.ua> In-Reply-To: <23856.1341389256.6316487571580649472@ffe17.ukr.net> References: <91943.1339669820.1305529125424791552@ffe15.ukr.net> <23856.1341389256.6316487571580649472@ffe17.ukr.net>
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--vX2ve0hYIA9A871f Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Jul 04, 2012 at 11:07:36AM +0300, Pavlo wrote: >=20 >=20 >=20 > --- Original message --- > From: "Pavlo" <devgs@ukr.net> > To: freebsd-fs@freebsd.org > Date: 14 June 2012, 13:30:20 > Subject: mmap() incoherency on hi I/O load (FS is zfs) >=20 >=20 > > There's a case when some parts of files that are mapped and then > modified getting corrupted. By corrupted I mean some data is ok (one that > was written using write()/pwrite()) but some looks like it never existed. > Like it was some time in buffers, when several processes simultaneously > (of course access was synchronised) used shared pages and reported it's > existence. But after time pass they (processes) screamed that it is now > lost. Only part of data written with pwrite() was there. Everything that > was written via mmap() is zero. > > > > So as I said it occurs on hi I/O busyness. When in background 4+ > processes do indexing of huge ammount of data. Also I want to note, it > never occurred in the life of our project while we used mmap() under > same I/O stress conditions when mapping was done for a whole file of just > a part(header) starting from a beginning of a file. First time we used > mapping of individual pages, just to save RAM, and this popped up. > > > > Solution for this problem is msync() before any munmap(). But man says: > > > > >=20 > The msync() system call is usually not needed since BSD implements a > coherent file system buffer cache. However, it may be used to associate > dirty VM pages with file system buffers and thus cause them to be flushed > to physical media sooner rather than later. > >=20 > > Any thoughts? Thanks. > >=20 > >=20 >=20 > So I tracked issue to the place where it occurs. When I commit data to > file using mmap() and pwrite() side by side, sometimes 'newest data' is > being overwritten by 'elder data'. From time to time 'elder data' can be > something written with mmap() either with pwrite(). It never happens when > I use exclusively mmap() either pwrite(). Also this issue reproduces on > UFS as well. I think there is a problem keeping mmapep pages and FS cache > synced. I am curious how do you label data with newer and elder labels. I do admit a possibility of a race in ZFS double-copy implementation of the mmap/cache coherency, but somewhat skeptical about the same possibility for UFS. What you saying might indicate that we loose modified/dirty bits for the page, but that would have much more firework then just eventual race with write. What version of the system ? Does the machine swap ? >=20 > I will try to make test to reliably reproduces issue. Yes, isolated test case is the best route forward. It would either show a bug or demonstrate a misunderstanding on your part. --vX2ve0hYIA9A871f Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAk/0B5kACgkQC3+MBN1Mb4gZ0QCg7SoPwIYcseI/gSsbOOyTboCN oxgAn0HWsYDFOdsxdsedeuEbucXyGPUc =DUC7 -----END PGP SIGNATURE----- --vX2ve0hYIA9A871f--
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