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>

1 files changed, 23 insertions, 1 deletions

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index c6083132c4b8..0387e05cdb98 100644
--- 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;
 }
 
 /*