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>

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index c6083132c4b8ccaf21c7addb61cf8596165bf915..0387e05cdb98b9708bde55143cd8a5cba853a2fa 100644 (file)
--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -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;
 }
 
 /*