linux-ext4 - Re: [PATCH 13/13] ext4: Support for synchronous DAX faults

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Date:   Thu, 24 Aug 2017 05:34:52 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-fsdevel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        Andy Lutomirski <luto@...nel.org>, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Boaz Harrosh <boazh@...app.com>
Subject: Re: [PATCH 13/13] ext4: Support for synchronous DAX faults

On Thu, Aug 24, 2017 at 05:31:26AM -0700, Christoph Hellwig wrote:
> On Thu, Aug 17, 2017 at 06:08:15PM +0200, Jan Kara wrote:
> > We return IOMAP_F_NEEDDSYNC flag from ext4_iomap_begin() for a
> > synchronous write fault when inode has some uncommitted metadata
> > changes. In the fault handler ext4_dax_fault() we then detect this case,
> > call vfs_fsync_range() to make sure all metadata is committed, and call
> > dax_pfn_mkwrite() to mark PTE as writeable. Note that this will also
> > dirty corresponding radix tree entry which is what we want - fsync(2)
> > will still provide data integrity guarantees for applications not using
> > userspace flushing. And applications using userspace flushing can avoid
> > calling fsync(2) and thus avoid the performance overhead.
> 
> Why is this only wiered up for the huge_fault handler and not the
> regular?

Ah, turns out ext4 implements ->fault in terms of ->huge_fault.

We'll really need to sort out this mess of fault handlers before
doing too much surgery here..