linux-kernel - Re: [PATCH] filesystem-dax: Disable PMD support

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Message-ID: <20190628165417.GD4286@bombadil.infradead.org>
Date:   Fri, 28 Jun 2019 09:54:17 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>, Jan Kara <jack@...e.cz>,
        stable <stable@...r.kernel.org>,
        Robert Barror <robert.barror@...el.com>,
        Seema Pandit <seema.pandit@...el.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] filesystem-dax: Disable PMD support

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@...radead.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).