Commit 502624bd authored May 10, 2013 by Mikulas Patocka Committed by Alasdair G Kergon May 10, 2013

dm bufio: avoid a possible __vmalloc deadlock



This patch uses memalloc_noio_save to avoid a possible deadlock in
dm-bufio.  (it could happen only with large block size, at most
PAGE_SIZE << MAX_ORDER (typically 8MiB).

__vmalloc doesn't fully respect gfp flags. The specified gfp flags are
used for allocation of requested pages, structures vmap_area, vmap_block
and vm_struct and the radix tree nodes.

However, the kernel pagetables are allocated always with GFP_KERNEL.
Thus the allocation of pagetables can recurse back to the I/O layer and
cause a deadlock.

This patch uses the function memalloc_noio_save to set per-process
PF_MEMALLOC_NOIO flag and the function memalloc_noio_restore to restore
it. When this flag is set, all allocations in the process are done with
implied GFP_NOIO flag, thus the deadlock can't happen.

This should be backported to stable kernels, but they don't have the
PF_MEMALLOC_NOIO flag and memalloc_noio_save/memalloc_noio_restore
functions. So, PF_MEMALLOC should be set and restored instead.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

parent 09e8b813

drivers/md/dm-bufio.c

+23 −1

Original line number	Diff line number	Diff line
		@@ -319,6 +319,9 @@ static void __cache_size_refresh(void)
		static void alloc_buffer_data(struct dm_bufio_client c, gfp_t gfp_mask,
		enum data_mode *data_mode)
		{
		unsigned noio_flag;
		void *ptr;

		if (c->block_size <= DM_BUFIO_BLOCK_SIZE_SLAB_LIMIT) {
		*data_mode = DATA_MODE_SLAB;
		return kmem_cache_alloc(DM_BUFIO_CACHE(c), gfp_mask);
		@@ -332,7 +335,26 @@ static void alloc_buffer_data(struct dm_bufio_client c, gfp_t gfp_mask,
		}

		*data_mode = DATA_MODE_VMALLOC;
		return __vmalloc(c->block_size, gfp_mask, PAGE_KERNEL);

		/*
		* __vmalloc allocates the data pages and auxiliary structures with
		* gfp_flags that were specified, but pagetables are always allocated
		* with GFP_KERNEL, no matter what was specified as gfp_mask.
		*
		* Consequently, we must set per-process flag PF_MEMALLOC_NOIO so that
		* all allocations done by this process (including pagetables) are done
		* as if GFP_NOIO was specified.
		*/

		if (gfp_mask & __GFP_NORETRY)
		noio_flag = memalloc_noio_save();

		ptr = __vmalloc(c->block_size, gfp_mask, PAGE_KERNEL);

		if (gfp_mask & __GFP_NORETRY)
		memalloc_noio_restore(noio_flag);

		return ptr;
		}

		/*

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