Re: [PATCH v2 5/9] mm/swap: introduce swapin_entry for unified readahead policy

[Kairui Song]

Huang, Ying <ying.huang@xxxxxxxxx> 于2024年1月5日周五 15:30写道：
>
> Kairui Song <ryncsn@xxxxxxxxx> writes:
>
> > From: Kairui Song <kasong@xxxxxxxxxxx>
> >
> > Introduce swapin_entry which merges swapin_readahead and swapin_direct
> > making it the main entry for swapin pages, and use a unified swapin
> > policy.
> >
> > This commit makes swapoff make use of this new helper and now swapping
> > off a 10G ZRAM (lzo-rle) is faster since readahead is skipped.
> >
> > Before:
> > time swapoff /dev/zram0
> > real    0m12.337s
> > user    0m0.001s
> > sys     0m12.329s
> >
> > After:
> > time swapoff /dev/zram0
> > real    0m9.728s
> > user    0m0.001s
> > sys     0m9.719s
> >
> > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> > ---
> >  mm/memory.c     | 21 +++++++--------------
> >  mm/swap.h       | 16 ++++------------
> >  mm/swap_state.c | 49 +++++++++++++++++++++++++++++++++----------------
> >  mm/swapfile.c   |  7 ++-----
> >  4 files changed, 46 insertions(+), 47 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 0165c8cad489..b56254a875f8 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3801,6 +3801,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       rmap_t rmap_flags = RMAP_NONE;
> >       bool exclusive = false;
> >       swp_entry_t entry;
> > +     bool swapcached;
> >       pte_t pte;
> >       vm_fault_t ret = 0;
> >
> > @@ -3864,21 +3865,13 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       swapcache = folio;
> >
> >       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 (folio)
> > -                             page = &folio->page;
> > +             folio = swapin_entry(entry, GFP_HIGHUSER_MOVABLE,
> > +                                  vmf, &swapcached);
> > +             if (folio) {
> > +                     page = folio_file_page(folio, swp_offset(entry));
> > +                     if (swapcached)
> > +                             swapcache = folio;
> >               } else {
> > -                     page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
> > -                                             vmf);
> > -                     if (page)
> > -                             folio = page_folio(page);
> > -                     swapcache = folio;
> > -             }
> > -
> > -             if (!folio) {
> >                       /*
> >                        * Back out if somebody else faulted in this pte
> >                        * while we released the pte lock.
> > diff --git a/mm/swap.h b/mm/swap.h
> > index 83eab7b67e77..502a2801f817 100644
> > --- a/mm/swap.h
> > +++ b/mm/swap.h
> > @@ -54,10 +54,8 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_flags,
> >               bool skip_if_exists);
> >  struct folio *swap_cluster_readahead(swp_entry_t entry, gfp_t flag,
> >               struct mempolicy *mpol, pgoff_t ilx);
> > -struct page *swapin_readahead(swp_entry_t entry, gfp_t flag,
> > -                           struct vm_fault *vmf);
> > -struct folio *swapin_direct(swp_entry_t entry, gfp_t flag,
> > -                         struct vm_fault *vmf);
> > +struct folio *swapin_entry(swp_entry_t entry, gfp_t flag,
> > +                         struct vm_fault *vmf, bool *swapcached);
> >
> >  static inline unsigned int folio_swap_flags(struct folio *folio)
> >  {
> > @@ -88,14 +86,8 @@ static inline struct folio *swap_cluster_readahead(swp_entry_t entry,
> >       return NULL;
> >  }
> >
> > -struct folio *swapin_direct(swp_entry_t entry, gfp_t flag,
> > -                     struct vm_fault *vmf)
> > -{
> > -     return NULL;
> > -}
> > -
> > -static inline struct page *swapin_readahead(swp_entry_t swp, gfp_t gfp_mask,
> > -                     struct vm_fault *vmf)
> > +static inline struct folio *swapin_entry(swp_entry_t swp, gfp_t gfp_mask,
> > +                     struct vm_fault *vmf, bool *swapcached)
> >  {
> >       return NULL;
> >  }
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index d39c5369da21..66ff187aa5d3 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -316,6 +316,11 @@ void free_pages_and_swap_cache(struct encoded_page **pages, int nr)
> >       release_pages(pages, nr);
> >  }
> >
> > +static inline bool swap_use_no_readahead(struct swap_info_struct *si, swp_entry_t entry)
> > +{
> > +     return data_race(si->flags & SWP_SYNCHRONOUS_IO) && __swap_count(entry) == 1;
> > +}
> > +

Hi Ying,

Thanks for the review.

>
> It appears that there's only one caller of the function in the same
> file?  Why add a function?

Later patch will extend the checker function.
I can defer this change so it won't cause confusion for reviewers.

>
> >  static inline bool swap_use_vma_readahead(void)
> >  {
> >       return READ_ONCE(enable_vma_readahead) && !atomic_read(&nr_rotate_swap);
> > @@ -870,8 +875,8 @@ static struct folio *swap_vma_readahead(swp_entry_t targ_entry, gfp_t gfp_mask,
> >   * Returns the struct folio for entry and addr after the swap entry is read
> >   * in.
> >   */
> > -struct folio *swapin_direct(swp_entry_t entry, gfp_t gfp_mask,
> > -                         struct vm_fault *vmf)
> > +static struct folio *swapin_direct(swp_entry_t entry, gfp_t gfp_mask,
> > +                               struct vm_fault *vmf)
> >  {
> >       struct vm_area_struct *vma = vmf->vma;
> >       struct folio *folio;
> > @@ -908,33 +913,45 @@ struct folio *swapin_direct(swp_entry_t entry, gfp_t gfp_mask,
> >  }
> >
> >  /**
> > - * swapin_readahead - swap in pages in hope we need them soon
> > + * swapin_entry - swap in a page from swap entry
> >   * @entry: swap entry of this memory
> >   * @gfp_mask: memory allocation flags
> >   * @vmf: fault information
> > + * @swapcached: pointer to a bool used as indicator if the
> > + *              page is swapped in through swapcache.
> >   *
> >   * Returns the struct page for entry and addr, after queueing swapin.
> >   *
> > - * It's a main entry function for swap readahead. By the configuration,
> > + * It's a main entry function for swap in. By the configuration,
> >   * it will read ahead blocks by cluster-based(ie, physical disk based)
> > - * or vma-based(ie, virtual address based on faulty address) readahead.
> > + * or vma-based(ie, virtual address based on faulty address) readahead,
> > + * or skip the readahead (ie, ramdisk based swap device).
> >   */
> > -struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> > -                             struct vm_fault *vmf)
> > +struct folio *swapin_entry(swp_entry_t entry, gfp_t gfp_mask,
> > +                        struct vm_fault *vmf, bool *swapcached)
>
> May be better to use
>
> struct folio *swapin_entry(swp_entry_t entry, gfp_t gfp_mask,
>                            struct vm_fault *vmf, struct folio **swapcache)
>
> In this way, we can reduce the number of source lines in the caller.

Following commit will rewrite this part to return a enum instead of
bool, so this is just a intermediate change. And do_swap_page is the
only caller that can benefit from this, not helpful for swapoff/shmem.
I think we can just keep it this way here.