linux-kernel - Re: [PATCH v4] mm/mempolicy,hugetlb: Checking hstate for hugetlbfs page in vma

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Date:   Tue, 21 Jan 2020 09:12:04 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Michal Hocko <mhocko@...nel.org>,
        Li Xinhai <lixinhai.lxh@...il.com>
Cc:     n-horiguchi <n-horiguchi@...jp.nec.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        akpm <akpm@...ux-foundation.org>,
        torvalds <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH v4] mm/mempolicy,hugetlb: Checking hstate for hugetlbfs
 page in vma_migratable



On 01/20/2020 09:35 PM, Michal Hocko wrote:
> On Mon 20-01-20 23:37:25, Li Xinhai wrote:
> [...]
>> Changelog is updated as below, thanks for comments:
>> ---
>> mm/mempolicy: Checking hugepage migration is supported by arch in vma_migratable
>>
>> vma_migratable() is called to check if pages in vma can be migrated
>> before go ahead to further actions. Currently it is used in below code
>> path:
>> - task_numa_work
>> - mbind
>> - move_pages
>>
>> For hugetlb mapping, whether vma is migratable or not is determined by:
>> - CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION
>> - arch_hugetlb_migration_supported
>>
>> Issue: current code only checks for CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION,
>> which express less accurate semantics of vma_migratable(). (note that
>> current code in vma_migratable don't cause failure or bug because
>> unmap_and_move_huge_page() will catch unsupported hugepage and handle it
>> properly)
>>
>> This patch checks the two factors for impoveing code logic and
>> robustness. It will enable early bail out of hugepage migration procedure,
>> but because currently all architecture supporting hugepage migration is able
>> to support all page size, we would not see performance gain with this patch
>> applied.
> 
> This looks definitely better than the original one. I hope it is more
> clear to you what I meant by a better description for the justification.
> I would just add that the no code should use
> CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION directly and use
> arch_hugetlb_migration_supported instead. This will be the case after
> this patch.

As I have mentioned previously on the other thread, there might be an case
to keep the existing code (just added with a comment) which will preserve
the performance. But the proposed method will do it the right way and also
get rid of CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION here. Its OK either way.

> 
> Please keep in mind that changelogs are really important and growing in
> importance as the code gets more complicated over time. It is much more
> easier to see what the patch does because reading diffs and the code is
> easy but the lack of motivation is what people usually fighting with.
>