On Tue, 8 Oct 2013 16:58:10 -0400 Johannes Weiner <han...@cmpxchg.org> wrote:
> Buffer allocation has a very crude indefinite loop around waking the > flusher threads and performing global NOFS direct reclaim because it > can not handle allocation failures. > > The most immediate problem with this is that the allocation may fail > due to a memory cgroup limit, where flushers + direct reclaim might > not make any progress towards resolving the situation at all. Because > unlike the global case, a memory cgroup may not have any cache at all, > only anonymous pages but no swap. This situation will lead to a > reclaim livelock with insane IO from waking the flushers and thrashing > unrelated filesystem cache in a tight loop. > > Use __GFP_NOFAIL allocations for buffers for now. This makes sure > that any looping happens in the page allocator, which knows how to > orchestrate kswapd, direct reclaim, and the flushers sensibly. It > also allows memory cgroups to detect allocations that can't handle > failure and will allow them to ultimately bypass the limit if reclaim > can not make progress. > > --- a/fs/buffer.c > +++ b/fs/buffer.c > @@ -1005,9 +1005,19 @@ grow_dev_page(struct block_device *bdev, sector_t > block, > struct buffer_head *bh; > sector_t end_block; > int ret = 0; /* Will call free_more_memory() */ > + gfp_t gfp_mask; > > - page = find_or_create_page(inode->i_mapping, index, > - (mapping_gfp_mask(inode->i_mapping) & ~__GFP_FS)|__GFP_MOVABLE); > + gfp_mask = mapping_gfp_mask(inode->i_mapping) & ~__GFP_FS; > + gfp_mask |= __GFP_MOVABLE; > + /* > + * XXX: __getblk_slow() can not really deal with failure and > + * will endlessly loop on improvised global reclaim. Prefer > + * looping in the allocator rather than here, at least that > + * code knows what it's doing. > + */ > + gfp_mask |= __GFP_NOFAIL; Yup. When I added GFP_NOFAIL all those years ago there were numerous open-coded try-forever loops, and GFP_NOFAIL was more a cleanup than anything else - move the loop into the page allocator, leaving behind a sentinel which says "this code sucks and should be fixed". Of course, nothing has since been fixed :( So apart from fixing a bug, this patch continues this conversion. I can't think why I didn't do it a decade ago! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
