Messages in this thread | | | Date | Mon, 4 Aug 2008 18:09:24 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: [PATCH] Export shmem_file_setup and shmem_getpage for DRM-GEM |
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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
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