Re: [PATCH 2/2] mm, thp: consolidate THP gfp handling into alloc_hugepage_direct_gfpmask

[Vlastimil Babka <vbabka@suse.cz>]

On 10/19/18 10:06 AM, Michal Hocko wrote:
> On Thu 18-10-18 19:11:47, Andrew Morton wrote:
>> On Wed, 26 Sep 2018 16:22:27 +0200 Michal Hocko <mhocko@kernel.org> wrote:
>>
>>>> MPOL_PREFERRED is handled by policy_node() before we call __alloc_pages_nodemask.
>>>> __GFP_THISNODE is applied only when we are not using
>>>> __GFP_DIRECT_RECLAIM which is handled in alloc_hugepage_direct_gfpmask
>>>> now.
>>>> Lastly MPOL_BIND wasn't handled explicitly but in the end the removed
>>>> late check would remove __GFP_THISNODE for it as well. So in the end we
>>>> are doing the same thing unless I miss something
>>>
>>> Forgot to add. One notable exception would be that the previous code
>>> would allow to hit
>>> 	WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE));
>>> in policy_node if the requested node (e.g. cpu local one) was outside of
>>> the mbind nodemask. This is not possible now. We haven't heard about any
>>> such warning yet so it is unlikely that it happens though.
>>
>> Perhaps a changelog addition is needed to cover the above?
> 
> : THP allocation mode is quite complex and it depends on the defrag
> : mode. This complexity is hidden in alloc_hugepage_direct_gfpmask from a
> : large part currently. The NUMA special casing (namely __GFP_THISNODE) is
> : however independent and placed in alloc_pages_vma currently. This both
> : adds an unnecessary branch to all vma based page allocation requests and
> : it makes the code more complex unnecessarily as well. Not to mention
> : that e.g. shmem THP used to do the node reclaiming unconditionally
> : regardless of the defrag mode until recently. This was not only
> : unexpected behavior but it was also hardly a good default behavior and I
> : strongly suspect it was just a side effect of the code sharing more than
> : a deliberate decision which suggests that such a layering is wrong.
> : 
> : Moreover the oriinal code allowed to trigger
> : 	WARN_ON_ONCE(policy->mode == MPOL_BIND && (gfp & __GFP_THISNODE));
> : in policy_node if the requested node (e.g. cpu local one) was outside of
> : the mbind nodemask. This is not possible now. We haven't heard about any
> : such warning yet so it is unlikely that it happens but still a signal of
> : a wrong code layering.

Ah, as I said in the other mail, I think it's inaccurate, the warning
was not possible to hit.

There's also a slight difference wrt MPOL_BIND. The previous code would
avoid using __GFP_THISNODE if the local node was outside of
policy_nodemask(). After your patch __GFP_THISNODE is avoided for all
MPOL_BIND policies. So there's a difference that if local node is
actually allowed by the bind policy's nodemask, previously
__GFP_THISNODE would be added, but now it won't be. I don't think it
matters that much though, but maybe the changelog could say that
(instead of the inaccurate note about warning). Note the other policy
where nodemask is relevant is MPOL_INTERLEAVE, and that's unchanged by
this patch.

When that's addressed, you can add

Acked-by: Vlastimil Babka <vbabka@suse.cz>

(Note I also agree with patch 1/2 but didn't think it was useful to
formally ack it on top of Mel's ack supported by actual measurements, as
we're all from the same company).

> : Get rid of the thp special casing from alloc_pages_vma and move the logic
> : to alloc_hugepage_direct_gfpmask. __GFP_THISNODE is applied to
> : the resulting gfp mask only when the direct reclaim is not requested and
> : when there is no explicit numa binding to preserve the current logic.
> : 
> : This allows for removing alloc_hugepage_vma as well.
> 
> Better?
>  
>> I assume that David's mbind() concern has gone away.
> 
> Either I've misunderstood it or it was not really a real issue.
>