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

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Message-ID: <CAPcyv4jQP-SFJGor-Q3VCRQ0xwt3MuVpH2qHx2wzyRA88DGQww@mail.gmail.com>
Date:   Thu, 27 Jun 2019 09:06:17 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
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 Thu, Jun 27, 2019 at 5:34 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Wed, Jun 26, 2019 at 05:15:45PM -0700, Dan Williams wrote:
> > Ever since the conversion of DAX to the Xarray a RocksDB benchmark has
> > been encountering intermittent lockups. The backtraces always include
> > the filesystem-DAX PMD path, multi-order entries have been a source of
> > bugs in the past, and disabling the PMD path allows a test that fails in
> > minutes to run for an hour.
>
> On May 4th, I asked you:
>
> Since this is provoked by a fatal signal, it must have something to do
> with a killable or interruptible sleep.  There's only one of those in the
> DAX code; fatal_signal_pending() in dax_iomap_actor().  Does rocksdb do
> I/O with write() or through a writable mmap()?  I'd like to know before
> I chase too far down this fault tree analysis.

RocksDB in this case is using write() for writes and mmap() for reads.