linux-ext4 - Re: [PATCH 17/17] xfs: support for synchronous DAX faults

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Message-ID: <20171027091301.GG31161@quack2.suse.cz>
Date:   Fri, 27 Oct 2017 11:13:01 +0200
From:   Jan Kara <jack@...e.cz>
To:     Christoph Hellwig <hch@....de>
Cc:     Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-ext4@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-api@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 17/17] xfs: support for synchronous DAX faults

On Fri 27-10-17 08:43:01, Christoph Hellwig wrote:
> On Thu, Oct 26, 2017 at 05:48:04PM +0200, Jan Kara wrote:
> > But now that I look at XFS implementation again, it misses handling
> > of VM_FAULT_NEEDSYNC in xfs_filemap_pfn_mkwrite() (ext4 gets this right).
> > I'll fix this by using __xfs_filemap_fault() for xfs_filemap_pfn_mkwrite()
> > as well since it mostly duplicates it anyway... Thanks for inquiring!
> 
> My first patches move xfs_filemap_pfn_mkwrite to use __xfs_filemap_fault,
> but that didn't work.  Wish I'd remember why, though.

Maybe due to the additional check on IS_DAX(inode) in __xfs_filemap_fault()
which could do the wrong thing if per-inode DAX flag is switched? Because
otherwise __xfs_filemap_fault(vmf, PE_SIZE_PTE, true) does exactly the same
thing as xfs_filemap_pfn_mkwrite() did.

If we do care about per-inode DAX flag switching, I can just fixup
xfs_filemap_pfn_mkwrite() but my understanding was that we ditched the idea
at least until someone comes up with a reliable way to implement that...

							Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR