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@xxxxxxxxxx> >> --- >> 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 */ >
