Re: [patch 2/2] fs: buffer: move allocation failure loop into the allocator

Andrew Morton
2013-12-04T00:59:15Z

On Tue, 8 Oct 2013 16:58:10 -0400 Johannes Weiner <hannes@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.

Problem.

> --- 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;

https://bugzilla.kernel.org/show_bug.cgi?id=65991

WARNING: CPU: 0 PID: 1 at mm/page_alloc.c:1539 get_page_from_freelist+0x8a9/0x8c0()
Modules linked in:
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.13.0-rc1 #42
Hardware name: Acer Aspire 7750G/JE70_HR, BIOS V1.07 03/02/2011
0000000000000009 ffff8801c6121650 ffffffff81898d39 0000000000000000
ffff8801c6121688 ffffffff8107dc43 0000000000000002 0000000000000001
0000000000284850 0000000000000000 ffff8801cec04680 ffff8801c6121698
Call Trace:
[<ffffffff81898d39>] dump_stack+0x4e/0x7a
[<ffffffff8107dc43>] warn_slowpath_common+0x73/0x90
[<ffffffff8107dd15>] warn_slowpath_null+0x15/0x20
[<ffffffff81116f69>] get_page_from_freelist+0x8a9/0x8c0
[<ffffffff81330cdd>] ? trace_hardirqs_off_thunk+0x3a/0x3c
[<ffffffff81117070>] __alloc_pages_nodemask+0xf0/0x770
[<ffffffff81330cdd>] ? trace_hardirqs_off_thunk+0x3a/0x3c
[<ffffffff81156823>] kmemcheck_alloc_shadow+0x53/0xf0
[<ffffffff81152495>] new_slab+0x345/0x3e0
[<ffffffff81897712>] __slab_alloc.isra.57+0x215/0x535
[<ffffffff81328030>] ? __radix_tree_preload+0x60/0xf0
[<ffffffff811545c8>] kmem_cache_alloc+0x118/0x150
[<ffffffff81328030>] ? __radix_tree_preload+0x60/0xf0
[<ffffffff81328030>] __radix_tree_preload+0x60/0xf0
[<ffffffff81328125>] radix_tree_maybe_preload+0x25/0x30
[<ffffffff8110faf7>] add_to_page_cache_locked+0x37/0x100
[<ffffffff8110fbd5>] add_to_page_cache_lru+0x15/0x40
[<ffffffff8110ff37>] find_or_create_page+0x57/0x90
[<ffffffff8118e630>] __getblk+0xf0/0x2f0

That __GFP_NOFAIL is getting down into
radix_tree_preload->kmem_cache_alloc() and I expect that in its
boundless stupidity, slab has decided to inappropriately go and use an
unnecessarily massive page size for radix_tree_node_cachep's underlying
memory allocations. So we end up using GFP_NOFAIL for an order=2 (or
more) allocation, which is unacceptably risky, methinks.

I really really wish slab wouldn't do this. The benefit is surely very
small and these unnecessary higher-order allocations are quite abusive
of the page allocator.

Can we please make slab stop doing this?

radix_tree_nodes are 560 bytes and the kernel often allocates them in
times of extreme memory stress. We really really want them to be
backed by order=0 pages.