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

[Matthew Wilcox]

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.

> Signed-off-by: Jianfeng Wang <jianfeng.w.wang@xxxxxxxxxx>
> ---
>  mm/mmap.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 1971bfffcc03..da0308eef435 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2330,7 +2330,10 @@ static void unmap_region(struct mm_struct *mm, struct ma_state *mas,
>  	struct mmu_gather tlb;
>  	unsigned long mt_start = mas->index;
>  
> +	/* Defer lru_add_drain() to tlb_finish_mmu() for the ifndef case. */
> +#ifdef CONFIG_MMU_GATHER_NO_GATHER
>  	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 +3303,10 @@ void exit_mmap(struct mm_struct *mm)
>  		return;
>  	}
>  
> +	/* Defer lru_add_drain() to tlb_finish_mmu() for the ifndef case. */
> +#ifdef CONFIG_MMU_GATHER_NO_GATHER
>  	lru_add_drain();
> +#endif
>  	flush_cache_mm(mm);
>  	tlb_gather_mmu_fullmm(&tlb, mm);
>  	/* update_hiwater_rss(mm) here? but nobody should be looking */
> -- 
> 2.42.1
> 
>