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

Huang, Ying
2024-03-27T06:32:39Z

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?

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?

> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 4dd894395a0f..ae8d3aa05df7 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3389,19 +3389,6 @@ int swapcache_prepare(swp_entry_t entry)
> return __swap_duplicate(entry, SWAP_HAS_CACHE);
> }
>
> -void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
> -{
> - struct swap_cluster_info *ci;
> - unsigned long offset = swp_offset(entry);
> - unsigned char usage;
> -
> - ci = lock_cluster_or_swap_info(si, offset);
> - usage = __swap_entry_free_locked(si, offset, SWAP_HAS_CACHE);
> - unlock_cluster_or_swap_info(si, ci);
> - if (!usage)
> - free_swap_slot(entry);
> -}
> -
> struct swap_info_struct *swp_swap_info(swp_entry_t entry)
> {
> return swap_type_to_swap_info(swp_type(entry));

--
Best Regards,
Huang, Ying