Re: [PATCH V2] mm: Introduce GFP_PGTABLE

From: Anshuman Khandual <anshuman.khandual@arm.com>
To: Michal Hocko <mhocko@kernel.org>, Matthew Wilcox <willy@infradead.org>
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Subject: Re: [PATCH V2] mm: Introduce GFP_PGTABLE
Date: Wed, 16 Jan 2019 18:57:13 +0530	[thread overview]
Message-ID: <d074a7aa-9582-b95a-dce0-d95ac3d3c949@arm.com> (raw)
In-Reply-To: <20190116124431.GK24149@dhcp22.suse.cz>

On 01/16/2019 06:14 PM, Michal Hocko wrote:
> On Wed 16-01-19 04:30:18, Matthew Wilcox wrote:
>> On Wed, Jan 16, 2019 at 07:57:03AM +0100, Michal Hocko wrote:
>>> On Wed 16-01-19 11:51:32, Anshuman Khandual wrote:
>>>> All architectures have been defining their own PGALLOC_GFP as (GFP_KERNEL |
>>>> __GFP_ZERO) and using it for allocating page table pages. This causes some
>>>> code duplication which can be easily avoided. GFP_KERNEL allocated and
>>>> cleared out pages (__GFP_ZERO) are required for page tables on any given
>>>> architecture. This creates a new generic GFP flag flag which can be used
>>>> for any page table page allocation. Does not cause any functional change.
>>>>
>>>> GFP_PGTABLE is being added into include/asm-generic/pgtable.h which is the
>>>> generic page tabe header just to prevent it's potential misuse as a general
>>>> allocation flag if included in include/linux/gfp.h.
>>>
>>> I haven't reviewed the patch yet but I am wondering whether this is
>>> really worth it without going all the way down to unify the common code
>>> and remove much more code duplication. Or is this not possible for some
>>> reason?
>>
>> Exactly what I suggested doing in response to v1.
>>
>> Also, the approach taken here is crazy.  x86 has a feature that no other
>> architecture has bothered to implement yet -- accounting page tables
>> to the process.  Yet instead of spreading that goodness to all other
>> architectures, Anshuman has gone to more effort to avoid doing that.
> 
> Yes, I believe the only reason this is x86 only is that each arch would
> have to be tweaked separately. So a cleanup in _that_ regard would be
> helpful. There is no real reason to have ptes accounted only for x86.
> There might be some exceptions but well, our asm-generic allows to opt
> in for generic implementation or override it with a special one. The
> later should be an exception rather than the rule.

Fair enough. So we seem to have agreement over __GFP_ACCOUNT for user page
tables but not for the kernel. But should we accommodate __GFP_RETRY_MAYFAIL
or drop them altogether (including multi order allocation requests) ?