On Wed, 2023-12-13 at 17:03 -0800, Jianfeng Wang wrote: > 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. Thanks for the explanation. Looking at the code, lru_add_drain() is executed for #ifndef CONFIG_MMU_GATHER_NO_GATHER in tlb_finish_mmu(). So the logic of your patch is fine. The #ifndef CONFIG_MMU_GATHER_NO_GATHER means mmu_gather feature is on. The double negative throws me off on in my first read of your commit log. Suggest that you add a comment in code to make it easier for future code maintenence: /* defer lru_add_drain() to tlb_finish_mmu() for ifndef CONFIG_MMU_GATHER_NO_GATHER */ Is your change of skipping the extra lru_add_drain() motivated by some performance reason in a workload? Wonder whether it is worth adding an extra ifdef in the code. Tim > > 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 */ > >
