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Date: Fri, 28 Jun 2019 09:54:17 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] filesystem-dax: Disable PMD support
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To: Dan Williams <dan.j.williams@intel.com>
Cc: Seema Pandit <seema.pandit@intel.com>, linux-nvdimm <linux-nvdimm@lists.01.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, stable <stable@vger.kernel.org>, Robert Barror <robert.barror@intel.com>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Jan Kara <jack@suse.cz>
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On Fri, Jun 28, 2019 at 09:39:01AM -0700, Dan Williams wrote:
> On Fri, Jun 28, 2019 at 9:37 AM Matthew Wilcox <willy@infradead.org> wrote:
> > That was the conclusion I came to; that one thread holding the mmap sem
> > for read isn't being woken up when it should be.  Just need to find it ...
> > obviously it's something to do with the PMD entries.
> 
> Can you explain to me one more time, yes I'm slow on the uptake on
> this, the difference between xas_load() and xas_find_conflict() and
> why it's ok for dax_lock_page() to use xas_load() while
> grab_mapping_entry() uses xas_find_conflict()?

When used with a non-zero 'order', xas_find_conflict() will return
an entry whereas xas_load() might return a pointer to a node.
dax_lock_page() always uses a zero order, so they would always do the
same thing (xas_find_conflict() would be less efficient).
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