Messages in this thread | | | From | Nick Piggin <> | Subject | Re: [PATCH] Export shmem_file_setup and shmem_getpage for DRM-GEM | Date | Tue, 5 Aug 2008 14:28:20 +1000 |
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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
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