Re: [PATCH] mm/mempolicy: fix mpol_rebind_nodemask() for MPOL_F_NUMA_BALANCING

[Jinjiang Tu <tujinjiang@huawei.com>]

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在 2025/12/15 8:04, Andrew Morton 写道:
> On Sat, 13 Dec 2025 16:29:11 +0800 Jinjiang Tu<tujinjiang@huawei.com> wrote:
>
>> When mempolicy is rebound due to the process moves to a different cpuset
>> context, or the set of nodes allowed by current cpuset context changes,
>> mpol_rebind_nodemask() remaps the nodemask according to the old and new
>> cpuset_mems_allowed by default. So, use mempolicy.w.cpuset_mems_allowed
>> to store the old nodemask allowed by cpuset.
>>
>> MPOL_F_STATIC_NODES suppresses the node remap and intersects the user's
>> passed nodemask and nodes allowed by new cpuset context.
>> For MPOL_F_RELATIVE_NODES, the user's passed nodemask means node IDs that
>> are relative to the set of node IDs allowed by the process's current
>> cpuset. So, use mempolicy.w.user_nodemask to store the user's passed
>> nodemask.
>>
>> commit bda420b98505 ("numa balancing: migrate on fault among multiple
>> bound nodes") adds new flag MPOL_F_NUMA_BALANCING to enable NUMA balancing
>> for MPOL_BIND, the behaviour of rebinding should be same with default
>> befaviour. However, mpol_store_user_nodemask() returns true for
>> MPOL_F_NUMA_BALANCING, leading to mempolicy.w.cpuset_mems_allowed stores
>> the user's passed nodemask instead of cpuset_current_mems_allowed, and
>> mpol_rebind_nodemask() remaps wrongly.
> Thanks.
>
> I find the changelog hard to follow, unfortunately.  It's odd that the
> problem description comes in the final paragraph!
>
> I cheekily changed that and then fed the text into Gemini, which
> I think helped. What do you think of the below?
>
Thanks, it describes the problem more clearly. I will update the
commit log in the next version.

> I won't merge the patch at this time - I'll await reviewer input.
>
>
> ## Bug Fix: Corrected `MPOL_BIND` Rebinding with `MPOL_F_NUMA_BALANCING`
>
> ### Problem
>
> The commit `bda420b98505` ("numa balancing: migrate on fault among
> multiple bound nodes") introduced the new flag
> **`MPOL_F_NUMA_BALANCING`** to enable NUMA balancing for the
> **`MPOL_BIND`** memory policy.
>
> The intended behavior was for the rebinding logic to remain the same as
> the default `MPOL_BIND` behavior.  However, the function
> `mpol_store_user_nodemask()` was incorrectly returning `true` for
> policies containing `MPOL_F_NUMA_BALANCING`.
>
> This led to a bug where:
>
> 1.  `mempolicy.w.cpuset_mems_allowed` stored the **user's passed
>      nodemask** instead of the actual nodemask allowed by the current
>      cpuset context (`cpuset_current_mems_allowed`).
>
> 2.  Consequently, **`mpol_rebind_nodemask()` performed incorrect
>      remapping** when the mempolicy was rebound.
>
> ### Analysis of Correct Rebinding Logic
>
> When a memory policy is rebound (e.g., because the process moves to a
> different cpuset context, or the allowed nodes within the current
> cpuset change), `mpol_rebind_nodemask()`, by default, remaps the
> policy's nodemask based on the transition between the **old** and
> **new** `cpuset_mems_allowed` sets.
>
> To support this mechanism correctly, `mempolicy.w.cpuset_mems_allowed`
> **must store the old nodemask allowed by the cpuset** before the
> transition.
>
> ### Context for Other Flags
>
> * **`MPOL_F_STATIC_NODES`**: This flag suppresses the node remap and
>    simply intersects the user's passed nodemask with the nodes allowed
>    by the new cpuset context.
>
> * **`MPOL_F_RELATIVE_NODES`**: For this policy, the user's passed
>    nodemask represents node IDs **relative** to the set of node IDs
>    allowed by the process's current cpuset.  Therefore,
>    `mempolicy.w.user_nodemask` is correctly used to store the user's
>    original relative nodemask.
>
> ### Proposed Fix
>
> Ensure that `mpol_store_user_nodemask()` handles
> `MPOL_F_NUMA_BALANCING` correctly so that
> `mempolicy.w.cpuset_mems_allowed` stores the correct cpuset-allowed
> nodemask, thereby restoring the proper remapping behavior in
> `mpol_rebind_nodemask()`.
>

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