linux-kernel - Re: [RFC 2/6] mm, mempolicy: stop adjusting current->il_next in mpol_rebind

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Date:   Tue, 11 Apr 2017 21:03:53 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Christoph Lameter <cl@...ux.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        David Rientjes <rientjes@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC 2/6] mm, mempolicy: stop adjusting current->il_next in
 mpol_rebind_nodemask()

On 11.4.2017 19:32, Christoph Lameter wrote:
> On Tue, 11 Apr 2017, Vlastimil Babka wrote:
> 
>> The task->il_next variable remembers the last allocation node for task's
>> MPOL_INTERLEAVE policy. mpol_rebind_nodemask() updates interleave and
>> bind mempolicies due to changing cpuset mems. Currently it also tries to
>> make sure that current->il_next is valid within the updated nodemask. This is
>> bogus, because 1) we are updating potentially any task's mempolicy, not just
>> current, and 2) we might be updating per-vma mempolicy, not task one.
>>
>> The interleave_nodes() function that uses il_next can cope fine with the value
>> not being within the currently allowed nodes, so this hasn't manifested as an
>> actual issue. Thus it also won't be an issue if we just remove this adjustment
>> completely.
> 
> Well, interleave_nodes() will then potentially return a node outside of
> the allowed memory policy when its called for the first time after
> mpol_rebind_.. . But thenn it will find the next node within the
> nodemask and work correctly for the next invocations.

Hmm, you're right. But that could be easily fixed if il_next became il_prev, so
we would return the result of next_node_in(il_prev) and also store it as the new
il_prev, right? I somehow assumed it already worked that way.

> But yea the race can probably be ignored. The idea was that the
> application has a stable memory footprint during rebinding.