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

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

On 2020-01-18 at 11:11 Li Xinhai wrote:
>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/1579147885-23511-1-git-send-email-lixinhai.lxh@gmail.com/

Add context information

>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 
- arch_hugetlb_migration_supported [1]
>
>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. 

[1] This interface is introduced by 
commit e693de186414 (mm/hugetlb: enable arch specific huge page
size support for migration), which allow improvement of this path.

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