Re: [patch] dax: fix PMD faults on zero-length files

From: Ross Zwisler <ross.zwisler@linux.intel.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	"Zwisler, Ross  <ross.zwisler@intel.com>,
	linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>
Subject: Re: [patch] dax: fix PMD faults on zero-length files
Date: Wed, 15 Nov 2017 12:16:37 -0700	[thread overview]
Message-ID: <20171115191637.GA30208@linux.intel.com> (raw)
In-Reply-To: <CAPcyv4gnqfBs9MyZtW0hq9Ffv8N5XXfpDYGP8GzCHnDGXe-EVA@mail.gmail.com>

On Tue, Nov 14, 2017 at 05:47:51PM -0800, Dan Williams wrote:
> On Tue, Nov 14, 2017 at 5:37 PM, Jeff Moyer <jmoyer@redhat.com> wrote:
> > PMD faults on a zero length file on a file system mounted with -o dax
> > will not generate SIGBUS as expected.
> >
> >         fd = open(...O_TRUNC);
> >         addr = mmap(NULL, 2*1024*1024, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
> >         *addr = 'a';
> >         <expect SIGBUS>
> >
> > The problem is this code in dax_iomap_pmd_fault:
> >
> >         max_pgoff = (i_size_read(inode) - 1) >> PAGE_SHIFT;
> >
> > If the inode size is zero, we end up with a max_pgoff that is way larger
> > than 0.  :)  Fix it by using DIV_ROUND_UP, as is done elsewhere in the
> > kernel.
> >
> > I tested this with some simple test code that ensured that SIGBUS was
> > received where expected.
> >
> 
> I assume this needs:
> 
>    Fixes: 642261ac995e ("dax: add struct iomap based DAX PMD support")
>    Cc: <stable@vger.kernel.org>
> 
> ...otherwise looks good to me.

Yep, this fix looks good, thanks.  You can add:

Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
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