linux-ext4 - Re: [PATCH 5/6] dax: Call ->iomap_begin without entry lock during dax fault

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Message-ID: <20161202100818.GA26086@quack2.suse.cz>
Date:   Fri, 2 Dec 2016 11:08:18 +0100
From:   Jan Kara <jack@...e.cz>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-mm@...ck.org,
        linux-nvdimm@...ts.01.org, Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 5/6] dax: Call ->iomap_begin without entry lock during
 dax fault

On Thu 01-12-16 15:24:47, Ross Zwisler wrote:
> On Thu, Nov 24, 2016 at 10:46:35AM +0100, Jan Kara wrote:
> > Currently ->iomap_begin() handler is called with entry lock held. If the
> > filesystem held any locks between ->iomap_begin() and ->iomap_end()
> > (such as ext4 which will want to hold transaction open), this would cause
> > lock inversion with the iomap_apply() from standard IO path which first
> > calls ->iomap_begin() and only then calls ->actor() callback which grabs
> > entry locks for DAX.
> 
> I don't see the dax_iomap_actor() grabbing any entry locks for DAX?  Is this
> an issue currently, or are you just trying to make the code consistent so we
> don't run into issues in the future?

So dax_iomap_actor() copies data from / to user provided buffer. That can
fault and if the buffer happens to be mmaped file on DAX filesystem, the
fault will end up grabbing entry locks. Sample evil test:

	fd = open("some_file", O_RDWR);
	buf = mmap(NULL, 65536, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
	write(fd, buf, 4096);

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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