Re: [PATCH] Export shmem_file_setup and shmem_getpage for DRM-GEM

[Hugh Dickins <hugh@veritas.com>]

On Mon, 4 Aug 2008, Keith Packard wrote:
> On Mon, 2008-08-04 at 20:43 +1000, Nick Piggin wrote:
> > read_mapping_page might help there.
> 
> That does look a lot more like what I want, as it returns an unlocked
> page. And, makes my code look cleaner to boot:
> 
>         inode = obj->filp->f_path.dentry->d_inode;
>         mapping = inode->i_mapping;
>         for (i = 0; i < page_count; i++) {
>                 page = read_mapping_page(mapping, i, NULL);
>                 if (IS_ERR(page)) {
>                         ret = PTR_ERR(page);
>                         DRM_ERROR("read_mapping_page failed: %d\n", ret);
>                         i915_gem_object_free_page_list(obj);
>                         return ret;
>                 }
>                 obj_priv->page_list[i] = page;
>         }
> 
> Does this look like it conforms to the vfs api? It appears to work when
> using shmem at least.

Yes, that's a good suggestion from Nick, should work with shmem/tmpfs
(with *proviso below) and others, and big improvement to your generality.

Whether such usage conforms to VFS API I'm not so sure: as I understand
it, it's really for internal use by a filesystem - if it's going to be
used beyond that, we ought to add a check that the filesystem it's used
upon really has a ->readpage method (and I'd rather we add such a check
than you do it at your end, in case we change the implementation later
to use something other than a ->readpage method - Nick, you'll be
nauseated to hear I was looking to see if ->fault with a pseudo-vma
could do it).  But if the layering police are happy with this, I am.

The *proviso is that for tmpfs itself this actually isn't as convenient
as directly using shmem_getpage: because this way passes a page in to
shmem_getpage, when maybe the page wanted is already here but currently
marked as swapcache rather than filecache.  It's an awkward extension
that allows tmpfs to support ->readpage at all.  But that route is in
use and well-tested, and only an inefficiency when swapping, so should
not cause you any problems.

(I have been agonizing over the way __read_cache_page, like your
original i915_gem_object_get_page_list, uses find_get_page: whereas
shmem_getpage uses find_lock_page.  I remember the latter is important,
but don't quite remember all the why.  I'm believing that it's really
only the ->fault usage where it becomes vital: things get confused, on
2.4 more than 2.6, if a swapcache tmpfs page is mapped into userspace.)

You don't examine page->mapping at all: good, there's a race by which
it could go to NULL after you acquired the page by read_mapping_page:
not by file truncation, but by swapping out at the wrong instant.
Don't worry, you have the right page, just don't rely on page->mapping.

(Sorry if I'm rather talking aloud to myself here.)

Hugh