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

[Nick Piggin <nickpiggin@yahoo.com.au>]

On Tuesday 05 August 2008 03:09, Hugh Dickins wrote:
> 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.

Yes, readpage is optional... A check will be required.

I don't exactly know if this would be deemed the best way to do it,
but it seems *relatively* filesystem agnostic (with the readpage
test being the exception), and it seems like it is to address_space
mappings what get_user_pages is to virtual memory mappings. And
there is no shortage of drivers using get_user_pages.

At any rate, it seems a bit nicer than using shmem_getpage directly.

cc'ing linux-fsdevel.


> 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