Re: [RFC PATCH 04/10] mm/swap: remove cache bypass swapin

[Kairui Song]

On Wed, Mar 27, 2024 at 2:32 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Kairui Song <ryncsn@xxxxxxxxx> writes:
>
> > From: Kairui Song <kasong@xxxxxxxxxxx>
> >
> > We used to have the cache bypass swapin path for better performance,
> > but by removing it, more optimization can be applied and have
> > an even better overall performance and less hackish.
> >
> > And these optimizations are not easily doable or not doable at all
> > without this.
> >
> > This patch simply removes it, and the performance will drop heavily
> > for simple swapin, things won't get this worse for real workloads
> > but still observable. Following commits will fix this and archive a
> > better performance.
> >
> > Swapout/in 30G zero pages from ZRAM (This mostly measures overhead
> > of swap path itself, because zero pages are not compressed but simply
> > recorded in ZRAM, and performance drops more as SWAP device is getting
> > full):
> >
> > Test result of sequential swapin/out:
> >
> >                Before (us)        After (us)
> > Swapout:       33619409           33624641
> > Swapin:        32393771           41614858 (-28.4%)
> > Swapout (THP): 7817909            7795530
> > Swapin (THP) : 32452387           41708471 (-28.4%)
> >
> > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> > ---
> >  mm/memory.c     | 18 ++++-------------
> >  mm/swap.h       | 10 +++++-----
> >  mm/swap_state.c | 53 ++++++++++---------------------------------------
> >  mm/swapfile.c   | 13 ------------
> >  4 files changed, 19 insertions(+), 75 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index dfdb620a9123..357d239ee2f6 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3932,7 +3932,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       struct page *page;
> >       struct swap_info_struct *si = NULL;
> >       rmap_t rmap_flags = RMAP_NONE;
> > -     bool need_clear_cache = false;
> >       bool exclusive = false;
> >       swp_entry_t entry;
> >       pte_t pte;
> > @@ -4000,14 +3999,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       if (!folio) {
> >               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
> >                   __swap_count(entry) == 1) {
> > -                     /* skip swapcache and readahead */
> >                       folio = swapin_direct(entry, GFP_HIGHUSER_MOVABLE, vmf);
> > -                     if (PTR_ERR(folio) == -EBUSY)
> > -                             goto out;
> > -                     need_clear_cache = true;
> >               } else {
> >                       folio = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE, vmf);
> > -                     swapcache = folio;
> >               }
> >
> >               if (!folio) {
> > @@ -4023,6 +4017,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >                       goto unlock;
> >               }
> >
> > +             swapcache = folio;
> >               page = folio_file_page(folio, swp_offset(entry));
> >
> >               /* Had to read the page from swap area: Major fault */
> > @@ -4187,7 +4182,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       vmf->orig_pte = pte;
> >
> >       /* ksm created a completely new copy */
> > -     if (unlikely(folio != swapcache && swapcache)) {
> > +     if (unlikely(folio != swapcache)) {
> >               folio_add_new_anon_rmap(folio, vma, vmf->address);
> >               folio_add_lru_vma(folio, vma);
> >       } else {
> > @@ -4201,7 +4196,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
> >
> >       folio_unlock(folio);
> > -     if (folio != swapcache && swapcache) {
> > +     if (folio != swapcache) {
> >               /*
> >                * Hold the lock to avoid the swap entry to be reused
> >                * until we take the PT lock for the pte_same() check
> > @@ -4227,9 +4222,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       if (vmf->pte)
> >               pte_unmap_unlock(vmf->pte, vmf->ptl);
> >  out:
> > -     /* Clear the swap cache pin for direct swapin after PTL unlock */
> > -     if (need_clear_cache)
> > -             swapcache_clear(si, entry);
> >       if (si)
> >               put_swap_device(si);
> >       return ret;
> > @@ -4240,12 +4232,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       folio_unlock(folio);
> >  out_release:
> >       folio_put(folio);
> > -     if (folio != swapcache && swapcache) {
> > +     if (folio != swapcache) {
> >               folio_unlock(swapcache);
> >               folio_put(swapcache);
> >       }
> > -     if (need_clear_cache)
> > -             swapcache_clear(si, entry);
> >       if (si)
> >               put_swap_device(si);
> >       return ret;
> > diff --git a/mm/swap.h b/mm/swap.h
> > index aee134907a70..ac9573b03432 100644
> > --- a/mm/swap.h
> > +++ b/mm/swap.h
> > @@ -41,7 +41,6 @@ void __delete_from_swap_cache(struct folio *folio,
> >  void delete_from_swap_cache(struct folio *folio);
> >  void clear_shadow_from_swap_cache(int type, unsigned long begin,
> >                                 unsigned long end);
> > -void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry);
> >  struct folio *swap_cache_get_folio(swp_entry_t entry,
> >               struct vm_area_struct *vma, unsigned long addr);
> >  struct folio *filemap_get_incore_folio(struct address_space *mapping,
> > @@ -100,14 +99,15 @@ static inline struct folio *swapin_readahead(swp_entry_t swp, gfp_t gfp_mask,
> >  {
> >       return NULL;
> >  }
> > -
> > -static inline int swap_writepage(struct page *p, struct writeback_control *wbc)
> > +static inline struct folio *swapin_direct(swp_entry_t entry, gfp_t flag,
> > +                     struct vm_fault *vmf);
> >  {
> > -     return 0;
> > +     return NULL;
> >  }
> >
> > -static inline void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
> > +static inline int swap_writepage(struct page *p, struct writeback_control *wbc)
> >  {
> > +     return 0;
> >  }
> >
> >  static inline struct folio *swap_cache_get_folio(swp_entry_t entry,
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index 2a9c6bdff5ea..49ef6250f676 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -880,61 +880,28 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
> >  }
> >
> >  /**
> > - * swapin_direct - swap in folios skipping swap cache and readahead
> > + * swapin_direct - swap in folios skipping readahead
> >   * @entry: swap entry of this memory
> >   * @gfp_mask: memory allocation flags
> >   * @vmf: fault information
> >   *
> > - * Returns the struct folio for entry and addr after the swap entry is read
> > - * in.
> > + * Returns the folio for entry after it is read in.
> >   */
> >  struct folio *swapin_direct(swp_entry_t entry, gfp_t gfp_mask,
> >                           struct vm_fault *vmf)
> >  {
> > -     struct vm_area_struct *vma = vmf->vma;
> > +     struct mempolicy *mpol;
> >       struct folio *folio;
> > -     void *shadow = NULL;
> > -
> > -     /*
> > -      * Prevent parallel swapin from proceeding with
> > -      * the cache flag. Otherwise, another thread may
> > -      * finish swapin first, free the entry, and swapout
> > -      * reusing the same entry. It's undetectable as
> > -      * pte_same() returns true due to entry reuse.
> > -      */
> > -     if (swapcache_prepare(entry)) {
> > -             /* Relax a bit to prevent rapid repeated page faults */
> > -             schedule_timeout_uninterruptible(1);
> > -             return ERR_PTR(-EBUSY);
> > -     }
> > -
> > -     /* skip swapcache */
> > -     folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
> > -                             vma, vmf->address, false);
> > -     if (folio) {
> > -             __folio_set_locked(folio);
> > -             __folio_set_swapbacked(folio);
> > -
> > -             if (mem_cgroup_swapin_charge_folio(folio,
> > -                                     vma->vm_mm, GFP_KERNEL,
> > -                                     entry)) {
> > -                     folio_unlock(folio);
> > -                     folio_put(folio);
> > -                     return NULL;
> > -             }
> > -             mem_cgroup_swapin_uncharge_swap(entry);
> > -
> > -             shadow = get_shadow_from_swap_cache(entry);
> > -             if (shadow)
> > -                     workingset_refault(folio, shadow);
> > +     bool page_allocated;
> > +     pgoff_t ilx;
> >
> > -             folio_add_lru(folio);
> > +     mpol = get_vma_policy(vmf->vma, vmf->address, 0, &ilx);
> > +     folio = __read_swap_cache_async(entry, gfp_mask, mpol, ilx,
> > +                                     &page_allocated, false);
> > +     mpol_cond_put(mpol);
> >
> > -             /* To provide entry to swap_read_folio() */
> > -             folio->swap = entry;
> > +     if (page_allocated)
> >               swap_read_folio(folio, true, NULL);
> > -             folio->private = NULL;
> > -     }
> >
> >       return folio;
> >  }
>
> This looks similar as read_swap_cache_async().  Can we merge them?

Yes, that's doable. But I may have to split it out again for later
optimizations though.

>
> And, we should avoid to readahead in swapin_readahead() or
> swap_vma_readahead() for SWP_SYNCHRONOUS_IO anyway.  So, it appears that
> we can change and use swapin_readahead() directly?

Good point, SWP_SYNCHRONOUS_IO check can be extended more after this
series, but readahead optimization could be another series (like the
previous one which tried to unify readahead for shmem/anon), so I
thought it's better to keep it untouched for now.