linux-kernel - Re: [PATCH] mm/mempolicy: Fix an incorrect rebind node in mpol_rebind

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Date:   Thu, 27 Jun 2019 11:59:54 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     zhong jiang <zhongjiang@...wei.com>, akpm@...ux-foundation.org,
        osalvador@...e.de, khandual@...ux.vnet.ibm.com, mhocko@...e.com,
        mgorman@...hsingularity.net, aarcange@...hat.com
Cc:     rcampbell@...dia.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mempolicy: Fix an incorrect rebind node in
 mpol_rebind_nodemask

On 5/25/19 9:07 AM, zhong jiang wrote:
> We bind an different node to different vma, Unluckily,
> it will bind different vma to same node by checking the /proc/pid/numa_maps.   
> Commit 213980c0f23b ("mm, mempolicy: simplify rebinding mempolicies when updating cpusets")
> has introduced the issue.  when we change memory policy by seting cpuset.mems,
> A process will rebind the specified policy more than one times. 
> if the cpuset_mems_allowed is not equal to user specified nodes. hence the issue will trigger.
> Maybe result in the out of memory which allocating memory from same node.

OK, how about this instead?

mpol_rebind_nodemask() is called for MPOL_BIND and MPOL_INTERLEAVE
mempoclicies when the tasks's cpuset's mems_allowed changes. For
policies created without MPOL_F_STATIC_NODES or MPOL_F_RELATIVE_NODES,
it works by remapping the policy's allowed nodes (stored in v.nodes)
using the previous value of mems_allowed (stored in
w.cpuset_mems_allowed) as the domain of map and the new mems_allowed
(passed as nodes) as the range of the map (see the comment of
bitmap_remap() for details).

The result of remapping is stored back as policy's nodemask in v.nodes,
and the new value of mems_allowed should be stored in
w.cpuset_mems_allowed to facilitate the next rebind, if it happens.

However, commit 213980c0f23b ("mm, mempolicy: simplify rebinding
mempolicies when updating cpusets") introduced a bug where the result of
remapping is stored in w.cpuset_mems_allowed instead. Thus, a
mempolicy's allowed nodes can evolve in an unexpected way after a series
of rebinding due to cpuset mems_allowed changes, possibly binding to a
wrong node or a smaller number of nodes which may e.g. overload them.
This patch fixes the bug so rebinding again works as intended.

> Fixes: 213980c0f23b ("mm, mempolicy: simplify rebinding mempolicies when updating cpusets") 
> Signed-off-by: zhong jiang <zhongjiang@...wei.com>

(an example of what exactly was the sequence of set_mempolicy and cpuset
mems changes with expected wrt actual results would be nice, but I think
the above should be fine by itself)

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/mempolicy.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index e3ab1d9..a60a3be 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -345,7 +345,7 @@ static void mpol_rebind_nodemask(struct mempolicy *pol, const nodemask_t *nodes)
>  	else {
>  		nodes_remap(tmp, pol->v.nodes,pol->w.cpuset_mems_allowed,
>  								*nodes);
> -		pol->w.cpuset_mems_allowed = tmp;
> +		pol->w.cpuset_mems_allowed = *nodes;
>  	}
>  
>  	if (nodes_empty(tmp))
>