linux-ext4 - Re: [PATCHSET 0/2] dax: fix broken pmem poison narrative

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Date:   Tue, 17 Aug 2021 08:46:01 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-xfs@...r.kernel.org, jane.chu@...cle.com,
        willy@...radead.org, tytso@....edu, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, sandeen@...deen.net
Subject: Re: [PATCHSET 0/2] dax: fix broken pmem poison narrative

On Tue, Aug 17, 2021 at 08:39:00AM +0100, Christoph Hellwig wrote:
> On Mon, Aug 16, 2021 at 02:05:18PM -0700, Darrick J. Wong wrote:
> > AFAICT, the only reason why the "punch and write" dance works at all is
> > that the XFS and ext4 currently call blkdev_issue_zeroout when
> > allocating pmem as part of a pwrite call.  A pwrite without the punch
> > won't clear the poison, because pwrite on a DAX file calls
> > dax_direct_access to access the memory directly, and dax_direct_access
> > is only smart enough to bail out on poisoned pmem.  It does not know how
> > to clear it.  Userspace could solve the problem by calling FIEMAP and
> > issuing a BLKZEROOUT, but that requires rawio capabilities.
> > 
> > The whole pmem poison recovery story is is wrong and needs to be
> > corrected ASAP before everyone else starts doing this.  Therefore,
> > create a dax_zeroinit_range function that filesystems can call to reset
> > the contents of the pmem to a known value and clear any state associated
> > with the media error.  Then, connect FALLOC_FL_ZERO_RANGE to this new
> > function (for DAX files) so that unprivileged userspace has a safe way
> > to reset the pmem and clear media errors.
> 
> I agree with the problem statement, but I don't think the fix is
> significantly better than what we have, as it still magically overloads
> other behavior.  I'd rather have an explicit operation to clear the
> poison both at the syscall level (maybe another falloc mode), and the
> internal kernel API level (new method in dax_operations).

I've long wondered why we can't just pass a write flag to the
direct_access functions so that pmem_dax_direct_access can clear the
poison.  Then we ought to be able to tell userspace that they can
recover from write errors by pwrite() or triggering a write fault on the
page, I think.  That's how userspace recovers from IO errors on
traditional disks; I've never understood why it has to be any different
now.

> Also for the next iteration please split the iomap changes from the
> usage in xfs.

ok.

--D