Patch "dm bufio: avoid a possible __vmalloc deadlock" has been added to the 3.4-stable tree

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This is a note to let you know that I've just added the patch titled

    dm bufio: avoid a possible __vmalloc deadlock

to the 3.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     dm-bufio-avoid-a-possible-__vmalloc-deadlock.patch
and it can be found in the queue-3.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From 4725f1715429f75fa5f053dbe05575d08aeb5967 Mon Sep 17 00:00:00 2001
From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Date: Fri, 10 May 2013 14:37:15 +0100
Subject: dm bufio: avoid a possible __vmalloc deadlock

From: Mikulas Patocka <mpatocka@xxxxxxxxxx>

commit 502624bdad3dba45dfaacaf36b7d83e39e74b2d2 upstream.

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@xxxxxxxxxx>
Signed-off-by: Alasdair G Kergon <agk@xxxxxxxxxx>
[bwh: Backported to 3.2 as recommended]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Cc: Rui Xiang <rui.xiang@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/md/dm-bufio.c |   26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -321,6 +321,9 @@ 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);
@@ -334,7 +337,28 @@ static void *alloc_buffer_data(struct dm
 	}
 
 	*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 = current->flags & PF_MEMALLOC;
+		current->flags |= PF_MEMALLOC;
+	}
+
+	ptr = __vmalloc(c->block_size, gfp_mask, PAGE_KERNEL);
+
+	if (gfp_mask & __GFP_NORETRY)
+		current->flags = (current->flags & ~PF_MEMALLOC) | noio_flag;
+
+	return ptr;
 }
 
 /*


Patches currently in stable-queue which might be from mpatocka@xxxxxxxxxx are

queue-3.4/dm-bufio-avoid-a-possible-__vmalloc-deadlock.patch
queue-3.4/dm-snapshot-avoid-snapshot-space-leak-on-crash.patch
queue-3.4/dm-snapshot-add-missing-module-aliases.patch
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