Re: [PATCH v2] mm: remove redundant lru_add_drain() prior to unmapping pages

[Jianfeng Wang]

On 12/14/23 7:06 PM, Matthew Wilcox wrote:
> On Thu, Dec 14, 2023 at 03:59:00PM -0800, Jianfeng Wang wrote:
>> On 12/14/23 3:00 PM, Matthew Wilcox wrote:
>>> On Thu, Dec 14, 2023 at 02:27:17PM -0800, Jianfeng Wang wrote:
>>>> When unmapping VMA pages, pages will be gathered in batch and released by
>>>> tlb_finish_mmu() if CONFIG_MMU_GATHER_NO_GATHER is not set. The function
>>>> tlb_finish_mmu() is responsible for calling free_pages_and_swap_cache(),
>>>> which calls lru_add_drain() to drain cached pages in folio_batch before
>>>> releasing gathered pages. Thus, it is redundant to call lru_add_drain()
>>>> before gathering pages, if CONFIG_MMU_GATHER_NO_GATHER is not set.
>>>>
>>>> Remove lru_add_drain() prior to gathering and unmapping pages in
>>>> exit_mmap() and unmap_region() if CONFIG_MMU_GATHER_NO_GATHER is not set.
>>>>
>>>> Note that the page unmapping process in oom_killer (e.g., in
>>>> __oom_reap_task_mm()) also uses tlb_finish_mmu() and does not have
>>>> redundant lru_add_drain(). So, this commit makes the code more consistent.
>>>
>>> Shouldn't we put this in __tlb_gather_mmu() which already has the
>>> CONFIG_MMU_GATHER_NO_GATHER ifdefs?  That would presuambly help with, eg
>>> zap_page_range_single() too.
>>>
>>
>> Thanks. It makes sense to me.
>> This commit is motivated by a workload that use mmap/unmap heavily.
>> While the mmu_gather feature is also used by hugetlb, madvise, mprotect,
>> etc., thus I prefer to have another standalone commit (following this one)
>> that moves lru_add_drain() to __tlb_gather_mmu() to unify these cases for
>> not making redundant lru_add_drain() calls when using mmu_gather.
> 
> That's not normally the approach we take.

Okay, understood. Thanks for pointing it out.
Let me send a new patch that aligns with your suggestion.