Re: [07/65] dm bufio: avoid a possible __vmalloc deadlock

Steven Rostedt
2013-06-05T14:54:33Z

On Tue, 2013-06-04 at 08:59 -0400, Mikulas Patocka wrote:
> > + */
> > +
> > + if (gfp_mask & __GFP_NORETRY) {
> > + noio_flag = current->flags;
>
> There should be noio_flag = current->flags & PF_MEMALLOC; because we don't
> want to restore other flags.
>

Here's the updated version:

Thanks,

-- Steve

>From e265a702cb479f58f451ec4458a26dd2bebeedab Mon Sep 17 00:00:00 2001
From: Mikulas Patocka <mpatocka@redhat.com>
Date: Fri, 10 May 2013 14:37:15 +0100
Subject: [PATCH] dm bufio: avoid a possible __vmalloc deadlock

[ Upstream commit 502624bdad3dba45dfaacaf36b7d83e39e74b2d2 ]

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>
[ Set and clear PF_MEMALLOC manually - SR ]
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
drivers/md/dm-bufio.c | 26 +++++++++++++++++++++++++-
1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index c0fc827..e9097af 100644
--- 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_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 = 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;
}

/*
--
1.7.10.4