Re: [LSF/MM TOPIC] Sharing file backed pages

[Amir Goldstein <amir73il@gmail.com>]

On Wed, Jan 23, 2019 at 4:54 PM Jan Kara <jack@suse.cz> wrote:
...
> >
> > At first glance, this requires dropping the assumption that a for an
> > uptodate clean page, vmf->vma->vm_file->f_inode == page->mapping->host.
> > Is there really such an assumption in common vfs/mm code?  and what will
> > it take to drop it?
>
> There definitely is such assumption. Take for example page reclaim as one
> such place that will be non-trivial to deal with. You need to remove the
> page from page cache of all inodes that contain it without having any file
> context whatsoever. So you will need to create some way for this page->page
> caches mapping to happen. Jerome in his talk at LSF/MM last year [1] actually
> nicely summarized what it would take to get rid of page->mapping
> dereferences. He even had some preliminary patches. To sum it up, it's a
> lot of intrusive work but in principle it is possible.
>
> [1] https://lwn.net/Articles/752564/
>

That would be real nice if that work makes progress.
However, for the sake of discussion, for the narrow case of overlayfs page
sharing, if page->mapping is the overlay mapping, then it already has
references to the underlying inode/mapping and overlayfs mapping ops
can do the right thing for reclaim and migrate.

So the fact that there is a lot of code referencing page->mapping (I know that)
doesn't really answer my question of how hard it is to drop the assumption
that vmf->vma->vm_file->f_inode == page->mapping->host for read protected
uptodate pages from common code.
Because if overlayfs (or any other arbitrator) will make sure that dirty pages
and non uptodate pages abide by existing page->mapping semantics, then
block layer code (for example) can still safely dereference page->mapping.

In any case, I'd really love to see the first part of Jerome's work merged, with
mapping propagated to all common helpers, even if the fs-specific patches
and KSM patches will take longer to land.

Thanks,
Amir.