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

Kairui Song
2024-03-27T06:56:11Z

On Wed, Mar 27, 2024 at 2:32 PM Huang, Ying <ying.huang@intel.com> wrote:
>
> Kairui Song <ryncsn@gmail.com> writes:
>
> > From: Kairui Song <kasong@tencent.com>
> >
> > 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@tencent.com>
> > ---
> > 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.