Re: [PATCH v4] mm/mempolicy,hugetlb: Checking hstate for hugetlbfs page in vma_migratable

["Li Xinhai" <lixinhai.lxh@gmail.com>]

On 2020-01-17 at 11:16 Li Xinhai wrote:
>On 2020-01-16 at 23:38 Li Xinhai wrote:
>>On 2020-01-16 at 23:18 Michal Hocko wrote:
>>>On Thu 16-01-20 21:50:34, Li Xinhai wrote:
>>>> On 2020-01-16 at 17:56 Michal Hocko wrote:
>>>> >On Thu 16-01-20 04:11:25, Li Xinhai wrote:
>>>> >> Checking hstate at early phase when isolating page, instead of during
>>>> >> unmap and move phase, to avoid useless isolation.
>>>> >
>>>> >Could you be more specific what you mean by isolation and why does it
>>>> >matter? The patch description should really explain _why_ the change is
>>>> >needed or desirable.
>>>>
>>>> The changelog can be improved:
>>>>
>>>> vma_migratable() is called to check if pages in vma can be migrated
>>>> before go ahead to isolate, unmap and move pages. For hugetlb pages,
>>>> hugepage_migration_supported(struct hstate *h) is one factor which
>>>> decide if migration is supported. In current code, this function is called
>>>> from unmap_and_move_huge_page(), after isolating page has
>>>> completed.
>>>> This patch checks hstate from vma_migratable() and avoids isolating pages
>>>> which are not supported.
>>>
>>>This still explains what but not why this is relevant. If by isolating
>>>pages you mean isolate_lru_page then this really a noop for hugetlb
>>>pages. Or do I still misread your changelog?
>>
>>I mean isolate_huge_page will queue pages for moving, and
>>unmap_and_move_huge_page will call
>>hugepage_migration_supported then refuse moving.
>>
>
>Forgot to mention that this patch has no relevant with this one
>https://patchwork.kernel.org/patch/11331639/, 
>
>Code change at here is common for avoids walking page table and
>isolate hugepage in case architecture or page size are not supported
>for migration. Comments from code are copied here:
>
>static int unmap_and_move_huge_page(...)
>{
>	/*
>	* Migratability of hugepages depends on architectures and their size.
>	* This check is necessary because some callers of hugepage migration
>	* like soft offline and memory hotremove don't walk through page
>	* tables or check whether the hugepage is pmd-based or not before
>	* kicking migration.
>	*/
>	if (!hugepage_migration_supported(page_hstate(hpage))) {
>	putback_active_hugepage(hpage);
>	return -ENOSYS;
>	}
>}
>
>For current code change, we are able to know the 'hstate' because we have 'vma', so
>do early check instead of later.
> 

https://lore.kernel.org/linux-mm/20200117111629898234212@gmail.com/

Revise with more details on changelog for reason why this patch
is need. Thanks for your comments.

---
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 (kernel\sched\fair.c)
- mbind (mm\mempolicy.c)
- move_pages (mm\migrate.c)

For hugetlb mapping, 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.

This patch checks the two factors to impove code logic. Besides, caller
of vma_migratable can take action properly in case architecture don't
support migration, e.g., mbind don't go further to try isolating/moving
pages, but currently it do continue because vma_migratable say yes.

No adding for Fix tag, since vma_migratable was implemented before
arch_hugetlb_migration_supported, it is up to the caller to use it.
---


>>>--
>>>Michal Hocko
>>>SUSE Labs