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

[Tim Chen]

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 */
> >