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

[Jianfeng Wang <jianfeng.w.wang@oracle.com>]

On 12/13/23 2:57 PM, Tim Chen wrote:
> On Tue, 2023-12-12 at 23:28 -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.
>>
>> Signed-off-by: Jianfeng Wang <jianfeng.w.wang@oracle.com>
>> ---
>>  mm/mmap.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 1971bfffcc03..0451285dee4f 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -2330,7 +2330,9 @@ static void unmap_region(struct mm_struct *mm, struct ma_state *mas,
>>  	struct mmu_gather tlb;
>>  	unsigned long mt_start = mas->index;
>>  
>> +#ifdef CONFIG_MMU_GATHER_NO_GATHER
> 
> In your comment you say skip lru_add_drain() when CONFIG_MMU_GATHER_NO_GATHER
> is *not* set.  So shouldn't this be
> 
> #ifndef CONFIG_MMU_GATHER_NO_GATHER ?
>
Hi Tim,

The mmu_gather feature is used to gather pages produced by unmap_vmas() and
release them in batch in tlb_finish_mmu(). The feature is *on* if
CONFIG_MMU_GATHER_NO_GATHER is *not* set. Note that: tlb_finish_mmu() will call
free_pages_and_swap_cache()/lru_add_drain() only when the feature is on.

Yes, this commit aims to skip lru_add_drain() when CONFIG_MMU_GATHER_NO_GATHER
is *not* set (i.e. when the mmu_gather feature is on) because it is redundant. 

If CONFIG_MMU_GATHER_NO_GATHER is set, pages will be released in unmap_vmas().
tlb_finish_mmu() will not call lru_add_drain(). So, it is still necessary to
keep the lru_add_drain() call to clear cached pages before unmap_vmas(), as
folio_batchs hold a reference count for pages in them.

The same applies to the other case.

Thanks,
- Jianfeng

>>  	lru_add_drain();
>> +#endif
>>  	tlb_gather_mmu(&tlb, mm);
>>  	update_hiwater_rss(mm);
>>  	unmap_vmas(&tlb, mas, vma, start, end, tree_end, mm_wr_locked);
>> @@ -3300,7 +3302,9 @@ void exit_mmap(struct mm_struct *mm)
>>  		return;
>>  	}
>>  
>> +#ifdef CONFIG_MMU_GATHER_NO_GATHER
> 
> same question as above.
> 
>>  	lru_add_drain();
>> +#endif
>>  	flush_cache_mm(mm);
>>  	tlb_gather_mmu_fullmm(&tlb, mm);
>>  	/* update_hiwater_rss(mm) here? but nobody should be looking */
>