Re: [PATCH RFC] mm: madvise: pageout: ignore references rather than clearing young

Barry Song
2024-02-23T22:20:48Z

On Sat, Feb 24, 2024 at 11:09 AM Minchan Kim <minchan@kernel.org> wrote:
>
> Hi Barry,
>
> On Fri, Feb 23, 2024 at 05:15:50PM +1300, Barry Song wrote:
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > While doing MADV_PAGEOUT, the current code will clear PTE young
> > so that vmscan won't read young flags to allow the reclamation
> > of madvised folios to go ahead.
>
> Isn't it good to accelerate reclaiming? vmscan checks whether the
> page was accessed recenlty by the young bit from pte and if it is,
> it doesn't reclaim the page. Since we have cleared the young bit
> in pte in madvise_pageout, vmscan is likely to reclaim the page
> since it wouldn't see the ferencecd_ptes from folio_check_references.

right, but the proposal is asking vmscan to skip the folio_check_references
if this is a PAGEOUT. so we remove both pte_clear_young and rmap
of folio_check_references.

>
> Could you clarify if I miss something here?

guest you missed we are skipping folio_check_references now.
we remove both, thus, make MADV_PAGEOUT 6% faster.

>
>
> > It seems we can do it by directly ignoring references, thus we
> > can remove tlb flush in madvise and rmap overhead in vmscan.
> >
> > Regarding the side effect, in the original code, if a parallel
> > thread runs side by side to access the madvised memory with the
> > thread doing madvise, folios will get a chance to be re-activated
> > by vmscan. But with the patch, they will still be reclaimed. But
> > this behaviour doing PAGEOUT and doing access at the same time is
> > quite silly like DoS. So probably, we don't need to care.
> >
> > A microbench as below has shown 6% decrement on the latency of
> > MADV_PAGEOUT,
> >
> > #define PGSIZE 4096
> > main()
> > {
> > int i;
> > #define SIZE 512*1024*1024
> > volatile long *p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
> > MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> >
> > for (i = 0; i < SIZE/sizeof(long); i += PGSIZE / sizeof(long))
> > p[i] = 0x11;
> >
> > madvise(p, SIZE, MADV_PAGEOUT);
> > }
> >
> > w/o patch w/ patch
> > root@10:~# time ./a.out root@10:~# time ./a.out
> > real 0m49.634s real 0m46.334s
> > user 0m0.637s user 0m0.648s
> > sys 0m47.434s sys 0m44.265s
> >
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > ---
> > mm/damon/paddr.c | 2 +-
> > mm/internal.h | 2 +-
> > mm/madvise.c | 8 ++++----
> > mm/vmscan.c | 12 +++++++-----
> > 4 files changed, 13 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
> > index 081e2a325778..5e6dc312072c 100644
> > --- a/mm/damon/paddr.c
> > +++ b/mm/damon/paddr.c
> > @@ -249,7 +249,7 @@ static unsigned long damon_pa_pageout(struct damon_region *r, struct damos *s)
> > put_folio:
> > folio_put(folio);
> > }
> > - applied = reclaim_pages(&folio_list);
> > + applied = reclaim_pages(&folio_list, false);
> > cond_resched();
> > return applied * PAGE_SIZE;
> > }
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 93e229112045..36c11ea41f47 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -868,7 +868,7 @@ extern unsigned long __must_check vm_mmap_pgoff(struct file *, unsigned long,
> > unsigned long, unsigned long);
> >
> > extern void set_pageblock_order(void);
> > -unsigned long reclaim_pages(struct list_head *folio_list);
> > +unsigned long reclaim_pages(struct list_head *folio_list, bool ignore_references);
> > unsigned int reclaim_clean_pages_from_list(struct zone *zone,
> > struct list_head *folio_list);
> > /* The ALLOC_WMARK bits are used as an index to zone->watermark */
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index abde3edb04f0..44a498c94158 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -386,7 +386,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> > return 0;
> > }
> >
> > - if (pmd_young(orig_pmd)) {
> > + if (!pageout && pmd_young(orig_pmd)) {
> > pmdp_invalidate(vma, addr, pmd);
> > orig_pmd = pmd_mkold(orig_pmd);
> >
> > @@ -410,7 +410,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> > huge_unlock:
> > spin_unlock(ptl);
> > if (pageout)
> > - reclaim_pages(&folio_list);
> > + reclaim_pages(&folio_list, true);
> > return 0;
> > }
> >
> > @@ -490,7 +490,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> >
> > VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
> >
> > - if (pte_young(ptent)) {
> > + if (!pageout && pte_young(ptent)) {
> > ptent = ptep_get_and_clear_full(mm, addr, pte,
> > tlb->fullmm);
> > ptent = pte_mkold(ptent);
> > @@ -524,7 +524,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> > pte_unmap_unlock(start_pte, ptl);
> > }
> > if (pageout)
> > - reclaim_pages(&folio_list);
> > + reclaim_pages(&folio_list, true);
> > cond_resched();
> >
> > return 0;
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 402c290fbf5a..ba2f37f46a73 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2102,7 +2102,8 @@ static void shrink_active_list(unsigned long nr_to_scan,
> > }
> >
> > static unsigned int reclaim_folio_list(struct list_head *folio_list,
> > - struct pglist_data *pgdat)
> > + struct pglist_data *pgdat,
> > + bool ignore_references)
> > {
> > struct reclaim_stat dummy_stat;
> > unsigned int nr_reclaimed;
> > @@ -2115,7 +2116,7 @@ static unsigned int reclaim_folio_list(struct list_head *folio_list,
> > .no_demotion = 1,
> > };
> >
> > - nr_reclaimed = shrink_folio_list(folio_list, pgdat, &sc, &dummy_stat, false);
> > + nr_reclaimed = shrink_folio_list(folio_list, pgdat, &sc, &dummy_stat, ignore_references);
> > while (!list_empty(folio_list)) {
> > folio = lru_to_folio(folio_list);
> > list_del(&folio->lru);
> > @@ -2125,7 +2126,7 @@ static unsigned int reclaim_folio_list(struct list_head *folio_list,
> > return nr_reclaimed;
> > }
> >
> > -unsigned long reclaim_pages(struct list_head *folio_list)
> > +unsigned long reclaim_pages(struct list_head *folio_list, bool ignore_references)
> > {
> > int nid;
> > unsigned int nr_reclaimed = 0;
> > @@ -2147,11 +2148,12 @@ unsigned long reclaim_pages(struct list_head *folio_list)
> > continue;
> > }
> >
> > - nr_reclaimed += reclaim_folio_list(&node_folio_list, NODE_DATA(nid));
> > + nr_reclaimed += reclaim_folio_list(&node_folio_list, NODE_DATA(nid),
> > + ignore_references);
> > nid = folio_nid(lru_to_folio(folio_list));
> > } while (!list_empty(folio_list));
> >
> > - nr_reclaimed += reclaim_folio_list(&node_folio_list, NODE_DATA(nid));
> > + nr_reclaimed += reclaim_folio_list(&node_folio_list, NODE_DATA(nid), ignore_references);
> >
> > memalloc_noreclaim_restore(noreclaim_flag);
> >
> > --
> > 2.34.1
> >

Thanks
Barry