linux-kernel - Re: [07/65] dm bufio: avoid a possible _

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Message-ID: <1370356941.26799.120.camel@gandalf.local.home>
Date:	Tue, 04 Jun 2013 10:42:21 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	stable@...nel.org, Alasdair G Kergon <agk@...hat.com>
Subject: Re: [07/65] dm bufio: avoid a possible __vmalloc deadlock

On Tue, 2013-06-04 at 08:59 -0400, Mikulas Patocka wrote:
> 
> On Mon, 3 Jun 2013, Steven Rostedt wrote:
> 
> > 3.6.11.5 stable review patch.
> > If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Mikulas Patocka <mpatocka@...hat.com>
> > 
> > [ 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@...hat.com>
> > Cc: stable@...nel.org
> > Signed-off-by: Alasdair G Kergon <agk@...hat.com>
> > [ Set and clear PF_MEMALLOC manually - SR ]
> > Signed-off-by: Steven Rostedt <rostedt@...dmis.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..a1f2487 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;
> 
> There should be noio_flag = current->flags & PF_MEMALLOC; because we don't 
> want to restore other flags.

Thanks for the review. Will fix.

-- Steve

> 
> > +		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
> 
> Mikulas


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