Re: [PATCH 5/7] fsdax: Dedup file range to use a compare function

Darrick J. Wong
2021-02-18T16:20:24Z

On Wed, Feb 17, 2021 at 11:24:18AM +0800, Ruan Shiyang wrote:
>
>
> On 2021/2/10 下午9:19, Christoph Hellwig wrote:
> > On Tue, Feb 09, 2021 at 05:46:13PM +0800, Ruan Shiyang wrote:
> > >
> > >
> > > On 2021/2/9 下午5:34, Christoph Hellwig wrote:
> > > > On Tue, Feb 09, 2021 at 05:15:13PM +0800, Ruan Shiyang wrote:
> > > > > The dax dedupe comparison need the iomap_ops pointer as argument, so my
> > > > > understanding is that we don't modify the argument list of
> > > > > generic_remap_file_range_prep(), but move its code into
> > > > > __generic_remap_file_range_prep() whose argument list can be modified to
> > > > > accepts the iomap_ops pointer. Then it looks like this:
> > > >
> > > > I'd say just add the iomap_ops pointer to
> > > > generic_remap_file_range_prep and do away with the extra wrappers. We
> > > > only have three callers anyway.
> > >
> > > OK.
> >
> > So looking at this again I think your proposal actaully is better,
> > given that the iomap variant is still DAX specific. Sorry for
> > the noise.
> >
> > Also I think dax_file_range_compare should use iomap_apply instead
> > of open coding it.
> >
>
> There are two files, which are not reflinked, need to be direct_access()
> here. The iomap_apply() can handle one file each time. So, it seems that
> iomap_apply() is not suitable for this case...
>
>
> The pseudo code of this process is as follows:
>
> srclen = ops->begin(&srcmap)
> destlen = ops->begin(&destmap)
>
> direct_access(&srcmap, &saddr)
> direct_access(&destmap, &daddr)
>
> same = memcpy(saddr, daddr, min(srclen,destlen))
>
> ops->end(&destmap)
> ops->end(&srcmap)
>
> I think a nested call like this is necessary. That's why I use the open
> code way.

This might be a good place to implement an iomap_apply2() loop that
actually /does/ walk all the extents of file1 and file2. There's now
two users of this idiom.

(Possibly structured as a "get next mappings from both" generator
function like Matthew Wilcox keeps asking for. :))

--D

>
> --
> Thanks,
> Ruan Shiyang.
> >