Re: [Lsf-pc] [LSF/MM TOPIC] sharing pages between mappings

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Jan 11, 2017 at 12:51:43PM +0100, Jan Kara wrote:
> On Wed 11-01-17 11:29:28, Miklos Szeredi wrote:
> > I know there's work on this for xfs, but could this be done in generic mm
> > code?
> > 
> > What are the obstacles?  page->mapping and page->index are the obvious
> > ones.
> 
> Yes, these two are the main that come to my mind. Also you'd need to
> somehow share the mapping->i_mmap tree so that unmap_mapping_range() works.
> 
> > If that's too difficult is it maybe enough to share mappings between
> > files while they are completely identical and clone the mapping when
> > necessary?
> 
> Well, but how would the page->mapping->host indirection work? Even if you
> have identical contents of the mappings, you still need to be aware there
> are several inodes behind them and you need to pick the right one
> somehow...
> 
> > All COW filesystems would benefit, as well as layered ones: lots of
> > fuse fs, and in some cases overlayfs too.
> > 
> > Related:  what can DAX do in the presence of cloned block?
> 
> For DAX handling a block COW should be doable if that is what you are
> asking about. Handling of blocks that can be written to while they are
> shared will be rather difficult (you have problems with keeping dirty bits
> in the radix tree consistent if nothing else).

I'm also interested in this topic, though I haven't gotten any further
than a hand-wavy notion of handling cow by allocating new blocks, memcpy
the contents to the new blocks (how?), then update the mappings to point
to the new blocks (how?).  It looks a lot easier now with the iomap
stuff, but that's as far as I got. :)

(IOWs it basically took all the time since the last LSF to get reflink
polished enough to handle regular files reasonably well.)

--D

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