Re: [PATCH] mm: numa: Do not trap faults on shared data section pages.

[Henry Willard <henry.willard@oracle.com>]

> On Jan 17, 2018, at 10:23 AM, Christopher Lameter <cl@linux.com> wrote:
> 
> On Tue, 16 Jan 2018, Mel Gorman wrote:
> 
>> My main source of discomfort is the fact that this is permanent as two
>> processes perfectly isolated but with a suitably shared COW mapping
>> will never migrate the data. A potential improvement to get the reported
>> bandwidth up in the test program would be to skip the rest of the VMA if
>> page_mapcount != 1 in a COW mapping as it would be reasonable to assume
>> the remaining pages in the VMA are also affected and the scan is wasteful.
>> There are counter-examples to this but I suspect that the full VMA being
>> shared is the common case. Whether you do that or not;
> 
> Same concern here. Typically CAP_SYS_NICE will bypass the check that the
> page is only mapped to a single process and the check looks exactly like
> the ones for manual migration. Using CAP_SYS_NICE would be surprising
> here since autonuma is not triggered by the currently running process.
> 
> Can we configure this somehow via sysfs?

If I understand the code correctly, CAP_SYS_NICE allows MPOL_MF_MOVE_ALL to be set with mbind() or used with move_pages(). CAP_SYS_NICE also causes migrate_pages() to behave as if MPOL_MF_MOVE_ALL were specified. There are checks requiring either MPOL_MF_MOVE_ALL or page_mapcount(page) == 1. The normal case does not call change_prot_numa(). change_prot_numa() is only called when MPOL_MF_LAZY is specified, and at the moment MPOL_MF_LAZY is not recognized as a valid flag. It looks to me that as things stand now, change_prot_numa() is only called from task_numa_work().

If MPOL_MF_LAZY were allowed and specified things would not work correctly. change_pte_range() is unaware of and can’t honor the difference between MPOL_MF_MOVE_ALL and MPOL_MF_MOVE. 

For the case of auto numa balancing, it may be undesirable for shared pages to be migrated whether they are also copy-on-write or not. The copy-on-write test was added to restrict the effect of the patch to the specific situation we observed. Perhaps I should remove it, I don’t understand why it would be desirable to modify the behavior via sysfs.

Thanks,
Henry
> 
> 

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