On Mon, Dec 15, 2008 at 03:21:44PM -0800, Andrew Morton wrote: > On Fri, 12 Dec 2008 05:41:20 +0100 > Nick Piggin <npiggin@xxxxxxx> wrote: > > > This patch doesn't actually fix a regression, but a longer standing bug. > > -- > > > > Frustratingly, gfp_t is really divided into two classes of flags. One are the > > context dependent ones (can we sleep? can we enter filesystem? block subsystem? > > should we use some extra reserves, etc.). The other ones are the type of memory > > required and depend on how the algorithm is implemented rather than the point > > at which the memory is allocated (highmem? dma memory? etc). > > > > Some of functions which allocate a page and add it to page cache take a gfp_t, > > but sometimes those functions or their callers aren't really doing the right > > thing: when allocating pagecache page, the memory type should be > > mapping_gfp_mask(mapping). When allocating radix tree nodes, the memory type > > should be kernel mapped (not highmem) memory. The gfp_t argument should only > > really be needed for context dependent options. > > > > This patch doesn't really solve that tangle in a nice way, but it does attempt > > to fix a couple of bugs. > > > > - find_or_create_page changes its radix-tree allocation to only include the > > main context dependent flags in order so the pagecache page may be allocated > > from arbitrary types of memory without affecting the radix-tree. In practice, > > slab allocations don't come from highmem anyway, and radix-tree only uses > > slab allocations. So there isn't a practical change (unless some fs uses > > GFP_DMA for pages). > > > > - grab_cache_page_nowait() is changed to allocate radix-tree nodes with > > GFP_NOFS, because it is not supposed to reenter the filesystem. This bug > > could cause lock recursion if a filesystem is not expecting the function > > to reenter the fs (as-per documentation). > > > > Filesystems should be careful about exactly what semantics they want and what > > they get when fiddling with gfp_t masks to allocate pagecache. One should be > > as liberal as possible with the type of memory that can be used, and same > > for the the context specific flags. > > ug. So at present page_symlink() can call write_begin() which will do > a GFP_KERNEL/GFP_USER allocation even though we hold fs locks? Yes. > In which calling context does this happen? ext3/4 do it AFAIKS. I think some filesystems set GFP_NOFS in mapping_gfp_mask too, but I don't think that's a very good idea because we don't always mask allocations with mapping_gfp_mask (but still, they would be helped by this patch too, if they were relying on it). > This is a pretty big ugly patch. I'm thinking that we merge into > 2.6.29 and backport into 2.6.28.x. Sounds good. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
