linux-ext4 - Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 5 May 2016 07:24:33 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Boaz Harrosh <boaz@...xistor.com>
Cc:	Vishal Verma <vishal.l.verma@...el.com>, linux-nvdimm@...1.01.org,
	Jens Axboe <axboe@...com>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <matthew@....cx>,
	Dave Chinner <david@...morbit.com>,
	linux-kernel@...r.kernel.org, xfs@....sgi.com,
	linux-block@...r.kernel.org, linux-mm@...ck.org,
	Al Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io

On Mon, May 02, 2016 at 06:41:51PM +0300, Boaz Harrosh wrote:
> > All IO in a dax filesystem used to go through dax_do_io, which cannot
> > handle media errors, and thus cannot provide a recovery path that can
> > send a write through the driver to clear errors.
> > 
> > Add a new iocb flag for DAX, and set it only for DAX mounts. In the IO
> > path for DAX filesystems, use the same direct_IO path for both DAX and
> > direct_io iocbs, but use the flags to identify when we are in O_DIRECT
> > mode vs non O_DIRECT with DAX, and for O_DIRECT, use the conventional
> > direct_IO path instead of DAX.
> > 
> 
> Really? What are your thinking here?
> 
> What about all the current users of O_DIRECT, you have just made them
> 4 times slower and "less concurrent*" then "buffred io" users. Since
> direct_IO path will queue an IO request and all.
> (And if it is not so slow then why do we need dax_do_io at all? [Rhetorical])
> 
> I hate it that you overload the semantics of a known and expected
> O_DIRECT flag, for special pmem quirks. This is an incompatible
> and unrelated overload of the semantics of O_DIRECT.

Agreed - makig O_DIRECT less direct than not having it is plain stupid,
and I somehow missed this initially.

This whole DAX story turns into a major nightmare, and I fear all our
hodge podge tweaks to the semantics aren't helping it.

It seems like we simply need an explicit O_DAX for the read/write
bypass if can't sort out the semantics (error, writer synchronization)
just as we need a special flag for MMAP..
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html