Message-ID: <87bk2bk87k.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Fri, 02 Aug 2024 09:46:23 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org,  baolin.wang@...ux.alibaba.com,
  chrisl@...nel.org,  david@...hat.com,  hannes@...xchg.org,
  hughd@...gle.com,  kaleshsingh@...gle.com,  kasong@...cent.com,
  linux-kernel@...r.kernel.org,  linux-mm@...ck.org,  mhocko@...e.com,
  minchan@...nel.org,  nphamcs@...il.com,  ryan.roberts@....com,
  senozhatsky@...omium.org,  shakeel.butt@...ux.dev,  shy828301@...il.com,
  surenb@...gle.com,  v-songbaohua@...o.com,  willy@...radead.org,
  xiang@...nel.org,  yosryahmed@...gle.com
Subject: Re: [PATCH 1/1] mm: swap: add nr argument in swapcache_prepare and
 swapcache_clear to support large folios

Barry Song <21cnbao@...il.com> writes:

> On Thu, Aug 1, 2024 at 2:50 PM Huang, Ying <ying.huang@...el.com> wrote:
>>
>> Barry Song <21cnbao@...il.com> writes:
>>
>> > On Thu, Aug 1, 2024 at 10:37 AM Huang, Ying <ying.huang@...el.com> wrote:
>> >>
>> >> Barry Song <21cnbao@...il.com> writes:
>> >>
>> >> > On Thu, Aug 1, 2024 at 9:13 AM Huang, Ying <ying.huang@...el.com> wrote:
>> >> >>
>> >> >> Barry Song <21cnbao@...il.com> writes:
>> >> >>
>> >> >> > On Wed, Jul 31, 2024 at 4:28 PM Huang, Ying <ying.huang@...el.com> wrote:
>> >> >> >>
>> >> >> >> Barry Song <21cnbao@...il.com> writes:
>> >> >> >>
>> >> >> >> > On Wed, Jul 31, 2024 at 4:14 PM Huang, Ying <ying.huang@...el.com> wrote:
>> >> >> >> >>
>> >> >> >> >> Hi, Barry,
>> >> >> >> >>
>> >> >> >> >> Barry Song <21cnbao@...il.com> writes:
>> >> >> >> >>
>> >> >> >> >> > From: Barry Song <v-songbaohua@...o.com>
>> >> >> >> >> >
>> >> >> >> >> > Right now, swapcache_prepare() and swapcache_clear() supports one entry
>> >> >> >> >> > only, to support large folios, we need to handle multiple swap entries.
>> >> >> >> >> >
>> >> >> >> >> > To optimize stack usage, we iterate twice in __swap_duplicate(): the
>> >> >> >> >> > first time to verify that all entries are valid, and the second time
>> >> >> >> >> > to apply the modifications to the entries.
>> >> >> >> >> >
>> >> >> >> >> > Currently, we're using nr=1 for the existing users.
>> >> >> >> >> >
>> >> >> >> >> > Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> >> >> >> >> > Signed-off-by: Barry Song <v-songbaohua@...o.com>
>> >> >> >> >> > ---
>> >> >> >> >> >  include/linux/swap.h |   4 +-
>> >> >> >> >> >  mm/memory.c          |   6 +--
>> >> >> >> >> >  mm/swap.h            |   5 ++-
>> >> >> >> >> >  mm/swap_state.c      |   2 +-
>> >> >> >> >> >  mm/swapfile.c        | 101 +++++++++++++++++++++++++------------------
>> >> >> >> >> >  5 files changed, 68 insertions(+), 50 deletions(-)
>> >> >> >> >> >
>> >> >> >> >> > diff --git a/include/linux/swap.h b/include/linux/swap.h
>> >> >> >> >> > index ba7ea95d1c57..5b920fa2315b 100644
>> >> >> >> >> > --- a/include/linux/swap.h
>> >> >> >> >> > +++ b/include/linux/swap.h
>> >> >> >> >> > @@ -480,7 +480,7 @@ extern int get_swap_pages(int n, swp_entry_t swp_entries[], int order);
>> >> >> >> >> >  extern int add_swap_count_continuation(swp_entry_t, gfp_t);
>> >> >> >> >> >  extern void swap_shmem_alloc(swp_entry_t);
>> >> >> >> >> >  extern int swap_duplicate(swp_entry_t);
>> >> >> >> >> > -extern int swapcache_prepare(swp_entry_t);
>> >> >> >> >> > +extern int swapcache_prepare(swp_entry_t entry, int nr);
>> >> >> >> >> >  extern void swap_free_nr(swp_entry_t entry, int nr_pages);
>> >> >> >> >> >  extern void swapcache_free_entries(swp_entry_t *entries, int n);
>> >> >> >> >> >  extern void free_swap_and_cache_nr(swp_entry_t entry, int nr);
>> >> >> >> >> > @@ -554,7 +554,7 @@ static inline int swap_duplicate(swp_entry_t swp)
>> >> >> >> >> >       return 0;
>> >> >> >> >> >  }
>> >> >> >> >> >
>> >> >> >> >> > -static inline int swapcache_prepare(swp_entry_t swp)
>> >> >> >> >> > +static inline int swapcache_prepare(swp_entry_t swp, int nr)
>> >> >> >> >> >  {
>> >> >> >> >> >       return 0;
>> >> >> >> >> >  }
>> >> >> >> >> > diff --git a/mm/memory.c b/mm/memory.c
>> >> >> >> >> > index 833d2cad6eb2..b8675617a5e3 100644
>> >> >> >> >> > --- a/mm/memory.c
>> >> >> >> >> > +++ b/mm/memory.c
>> >> >> >> >> > @@ -4081,7 +4081,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> >> >> >> >> >                        * reusing the same entry. It's undetectable as
>> >> >> >> >> >                        * pte_same() returns true due to entry reuse.
>> >> >> >> >> >                        */
>> >> >> >> >> > -                     if (swapcache_prepare(entry)) {
>> >> >> >> >> > +                     if (swapcache_prepare(entry, 1)) {
>> >> >> >> >> >                               /* Relax a bit to prevent rapid repeated page faults */
>> >> >> >> >> >                               schedule_timeout_uninterruptible(1);
>> >> >> >> >> >                               goto out;
>> >> >> >> >> > @@ -4387,7 +4387,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> >> >> >> >> >  out:
>> >> >> >> >> >       /* Clear the swap cache pin for direct swapin after PTL unlock */
>> >> >> >> >> >       if (need_clear_cache)
>> >> >> >> >> > -             swapcache_clear(si, entry);
>> >> >> >> >> > +             swapcache_clear(si, entry, 1);
>> >> >> >> >> >       if (si)
>> >> >> >> >> >               put_swap_device(si);
>> >> >> >> >> >       return ret;
>> >> >> >> >> > @@ -4403,7 +4403,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> >> >> >> >> >               folio_put(swapcache);
>> >> >> >> >> >       }
>> >> >> >> >> >       if (need_clear_cache)
>> >> >> >> >> > -             swapcache_clear(si, entry);
>> >> >> >> >> > +             swapcache_clear(si, entry, 1);
>> >> >> >> >> >       if (si)
>> >> >> >> >> >               put_swap_device(si);
>> >> >> >> >> >       return ret;
>> >> >> >> >> > diff --git a/mm/swap.h b/mm/swap.h
>> >> >> >> >> > index baa1fa946b34..7c6330561d84 100644
>> >> >> >> >> > --- a/mm/swap.h
>> >> >> >> >> > +++ b/mm/swap.h
>> >> >> >> >> > @@ -59,7 +59,7 @@ 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);
>> >> >> >> >> > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry, int nr);
>> >> >> >> >> >  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,
>> >> >> >> >> > @@ -120,7 +120,7 @@ static inline int swap_writepage(struct page *p, struct writeback_control *wbc)
>> >> >> >> >> >       return 0;
>> >> >> >> >> >  }
>> >> >> >> >> >
>> >> >> >> >> > -static inline void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
>> >> >> >> >> > +static inline void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry, int nr)
>> >> >> >> >> >  {
>> >> >> >> >> >  }
>> >> >> >> >> >
>> >> >> >> >> > @@ -172,4 +172,5 @@ static inline unsigned int folio_swap_flags(struct folio *folio)
>> >> >> >> >> >       return 0;
>> >> >> >> >> >  }
>> >> >> >> >> >  #endif /* CONFIG_SWAP */
>> >> >> >> >> > +
>> >> >> >> >>
>> >> >> >> >> NITPICK: Is it necessary to add a blank line here?  But I don't think a
>> >> >> >> >> new version is necessary if this is the only change needed.
>> >> >> >> >
>> >> >> >> > No need to add a blank line; it was probably a mistake I made in Vim.
>> >> >> >> >
>> >> >> >> >>
>> >> >> >> >> >  #endif /* _MM_SWAP_H */
>> >> >> >> >> > diff --git a/mm/swap_state.c b/mm/swap_state.c
>> >> >> >> >> > index a1726e49a5eb..b06f2a054f5a 100644
>> >> >> >> >> > --- a/mm/swap_state.c
>> >> >> >> >> > +++ b/mm/swap_state.c
>> >> >> >> >> > @@ -477,7 +477,7 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>> >> >> >> >> >               /*
>> >> >> >> >> >                * Swap entry may have been freed since our caller observed it.
>> >> >> >> >> >                */
>> >> >> >> >> > -             err = swapcache_prepare(entry);
>> >> >> >> >> > +             err = swapcache_prepare(entry, 1);
>> >> >> >> >> >               if (!err)
>> >> >> >> >> >                       break;
>> >> >> >> >> >
>> >> >> >> >> > diff --git a/mm/swapfile.c b/mm/swapfile.c
>> >> >> >> >> > index 5f73a8553371..757d38a86f56 100644
>> >> >> >> >> > --- a/mm/swapfile.c
>> >> >> >> >> > +++ b/mm/swapfile.c
>> >> >> >> >> > @@ -3363,7 +3363,7 @@ void si_swapinfo(struct sysinfo *val)
>> >> >> >> >> >  }
>> >> >> >> >> >
>> >> >> >> >> >  /*
>> >> >> >> >> > - * Verify that a swap entry is valid and increment its swap map count.
>> >> >> >> >> > + * Verify that nr swap entries are valid and increment their swap map counts.
>> >> >> >> >> >   *
>> >> >> >> >> >   * Returns error code in following case.
>> >> >> >> >> >   * - success -> 0
>> >> >> >> >> > @@ -3373,60 +3373,77 @@ void si_swapinfo(struct sysinfo *val)
>> >> >> >> >> >   * - swap-cache reference is requested but the entry is not used. -> ENOENT
>> >> >> >> >> >   * - swap-mapped reference requested but needs continued swap count. -> ENOMEM
>> >> >> >> >> >   */
>> >> >> >> >> > -static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
>> >> >> >> >> > +static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
>> >> >> >> >> >  {
>> >> >> >> >> >       struct swap_info_struct *p;
>> >> >> >> >> >       struct swap_cluster_info *ci;
>> >> >> >> >> >       unsigned long offset;
>> >> >> >> >> >       unsigned char count;
>> >> >> >> >> >       unsigned char has_cache;
>> >> >> >> >> > -     int err;
>> >> >> >> >> > +     int err, i;
>> >> >> >> >> >
>> >> >> >> >> >       p = swp_swap_info(entry);
>> >> >> >> >> >
>> >> >> >> >> >       offset = swp_offset(entry);
>> >> >> >> >> > +     VM_WARN_ON(nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER);
>> >> >> >> >> >       ci = lock_cluster_or_swap_info(p, offset);
>> >> >> >> >> >
>> >> >> >> >> > -     count = p->swap_map[offset];
>> >> >> >> >> > +     err = 0;
>> >> >> >> >> > +     for (i = 0; i < nr; i++) {
>> >> >> >> >> > +             count = p->swap_map[offset + i];
>> >> >> >> >> >
>> >> >> >> >> > -     /*
>> >> >> >> >> > -      * swapin_readahead() doesn't check if a swap entry is valid, so the
>> >> >> >> >> > -      * swap entry could be SWAP_MAP_BAD. Check here with lock held.
>> >> >> >> >> > -      */
>> >> >> >> >> > -     if (unlikely(swap_count(count) == SWAP_MAP_BAD)) {
>> >> >> >> >> > -             err = -ENOENT;
>> >> >> >> >> > -             goto unlock_out;
>> >> >> >> >> > -     }
>> >> >> >> >> > +             /*
>> >> >> >> >> > +              * swapin_readahead() doesn't check if a swap entry is valid, so the
>> >> >> >> >> > +              * swap entry could be SWAP_MAP_BAD. Check here with lock held.
>> >> >> >> >> > +              */
>> >> >> >> >> > +             if (unlikely(swap_count(count) == SWAP_MAP_BAD)) {
>> >> >> >> >> > +                     err = -ENOENT;
>> >> >> >> >> > +                     goto unlock_out;
>> >> >> >> >> > +             }
>> >> >> >> >> >
>> >> >> >> >> > -     has_cache = count & SWAP_HAS_CACHE;
>> >> >> >> >> > -     count &= ~SWAP_HAS_CACHE;
>> >> >> >> >> > -     err = 0;
>> >> >> >> >> > +             has_cache = count & SWAP_HAS_CACHE;
>> >> >> >> >> > +             count &= ~SWAP_HAS_CACHE;
>> >> >> >> >> >
>> >> >> >> >> > -     if (usage == SWAP_HAS_CACHE) {
>> >> >> >> >> > +             if (usage == SWAP_HAS_CACHE) {
>> >> >> >> >> > +                     /* set SWAP_HAS_CACHE if there is no cache and entry is used */
>> >> >> >> >> > +                     if (!has_cache && count)
>> >> >> >> >> > +                             continue;
>> >> >> >> >> > +                     else if (has_cache)             /* someone else added cache */
>> >> >> >> >> > +                             err = -EEXIST;
>> >> >> >> >> > +                     else                            /* no users remaining */
>> >> >> >> >> > +                             err = -ENOENT;
>> >> >> >> >> >
>> >> >> >> >> > -             /* set SWAP_HAS_CACHE if there is no cache and entry is used */
>> >> >> >> >> > -             if (!has_cache && count)
>> >> >> >> >> > -                     has_cache = SWAP_HAS_CACHE;
>> >> >> >> >> > -             else if (has_cache)             /* someone else added cache */
>> >> >> >> >> > -                     err = -EEXIST;
>> >> >> >> >> > -             else                            /* no users remaining */
>> >> >> >> >> > -                     err = -ENOENT;
>> >> >> >> >> > +             } else if (count || has_cache) {
>> >> >> >> >> >
>> >> >> >> >> > -     } else if (count || has_cache) {
>> >> >> >> >> > +                     if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
>> >> >> >> >> > +                             continue;
>> >> >> >> >> > +                     else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
>> >> >> >> >> > +                             err = -EINVAL;
>> >> >> >> >> > +                     else if (swap_count_continued(p, offset + i, count))
>> >> >> >> >> > +                             continue;
>> >> >> >> >>
>> >> >> >> >> IIUC, this will make the change to swap map directly instead of
>> >> >> >> >> verification.  If the verification failed for some entry later, the
>> >> >> >> >> count will be wrong?  Or I missed something?
>> >> >> >> >
>> >> >> >> > To avoid using a bitmap or a larger stack, we actually verify during
>> >> >> >> > the first iteration.
>> >> >> >> > This ensures that by the second iteration, we can safely commit the
>> >> >> >> > modification.
>> >> >> >> >
>> >> >> >> > I actually put some words in the changelog :-)
>> >> >> >> >
>> >> >> >> > To optimize stack usage, we iterate twice in __swap_duplicate(): the
>> >> >> >> > first time to verify that all entries are valid, and the second time
>> >> >> >> > to apply the modifications to the entries.
>> >> >> >>
>> >> >> >> Yes, I have seen it and I think that it is a good strategy.
>> >> >> >>
>> >> >> >> But, IIUC, swap_count_continued() will change the higher bits of the
>> >> >> >> swap_map instead of verifying.  Or, my understanding is wrong?
>> >> >> >>
>> >> >> >
>> >> >> > Ying, your understanding is 100% correct. but the code also has nothing
>> >> >> > broken. we didn't extend swap_duplicate() to have argument nr,
>> >> >> > so all users which can set usage=1 will definitely have nr=1.
>> >> >> >
>> >> >> > int swap_duplicate(swp_entry_t entry)
>> >> >> > {
>> >> >> >         int err = 0;
>> >> >> >
>> >> >> >         while (!err && __swap_duplicate(entry, 1, 1) == -ENOMEM)
>> >> >> >                 err = add_swap_count_continuation(entry, GFP_ATOMIC);
>> >> >> >         return err;
>> >> >> > }
>> >> >>
>> >> >> I understand that we don't have requirements to support "usage == 1 &&
>> >> >> nr > 1" case for __swap_duplicate() at least for now.
>> >> >>
>> >> >> > Maybe I can add a VM_WARN_ON to warn those people who might
>> >> >> > want to extend swap_duplicate()? in that case, things could be quite
>> >> >> > tricky.
>> >> >> >
>> >> >> > --- a/mm/swapfile.c
>> >> >> > +++ b/mm/swapfile.c
>> >> >> > @@ -3386,6 +3386,7 @@ static int __swap_duplicate(swp_entry_t entry,
>> >> >> > unsigned char usage, int nr)
>> >> >> >
>> >> >> >         offset = swp_offset(entry);
>> >> >> >         VM_WARN_ON(nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER);
>> >> >> > +       VM_WARN_ON(usage == 1 && nr > 1);
>> >> >> >         ci = lock_cluster_or_swap_info(p, offset);
>> >> >> >
>> >> >> >         err = 0;
>> >> >>
>> >> >> Please add this.  And, I think that we need to make it explicit in patch
>> >> >> description and comments to avoid potential confusing.
>> >> >
>> >> > cool. make sense to me. I will post something for Andrew to squash into.
>> >> >
>> >> >>
>> >> >> And, because it's hard to implement the verify and change strategy if
>> >> >> "usage == 1".  Can we only use that strategy for "usage ==
>> >> >> SWAP_HAS_CACHE"?
>> >> >
>> >> > I believe Baolin also needs the case for shmem. I don't feel a strong
>> >> > need to split two logics(1 and non-1) as the code will be quite ugly :-)
>> >>
>> >> Don't need to split like that, it could be something like
>> >>
>> >>         for (i = 0; i < nr; i++) {
>> >>                 if (usage == SWAP_HAS_CACHE) {
>> >>                         /* Only verify for SWAP_HAS_CACHE */
>> >>                 }
>> >>         }
>> >>
>> >>         for (i = 0; i < nr; i++) {
>> >>                 if (usage == SWAP_HAS_CACHE) {
>> >>                 } else {
>> >>                         /* Verify and change for usage == 1 */
>> >>                 }
>> >>         }
>> >>
>> >
>> > but we also have cases where nr can be > 1
>> >         __swap_duplicate(entry, SWAP_MAP_SHMEM, 1);
>>
>> If we can do verification for "usage == SWAP_MAP_SHMEM", we can add that
>> in the first loop.
>>
>> That is, we only do verification in the first loop, not do committing.
>> In the second loop, we can ignore verifying if we have done that in the
>> first loop.
>>
>> IMHO, this make code easier to be understood.
>
> Right. I believe the change below can help improve readability and also
> clarify the swap_count_continued() case.
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 2fa29bdec171..75a89ce18edc 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3538,6 +3538,7 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
>  
>  	offset = swp_offset(entry);
>  	VM_WARN_ON(nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER);
> +	VM_WARN_ON(usage == 1 && nr > 1);
>  	ci = lock_cluster_or_swap_info(p, offset);
>  
>  	err = 0;
> @@ -3564,17 +3565,9 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
>  				err = -EEXIST;
>  			else				/* no users remaining */
>  				err = -ENOENT;
> -
>  		} else if (count || has_cache) {
> -
> -			if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
> -				continue;
> -			else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
> +			if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
>  				err = -EINVAL;
> -			else if (swap_count_continued(p, offset + i, count))
> -				continue;
> -			else
> -				err = -ENOMEM;
>  		} else
>  			err = -ENOENT;			/* unused swap entry */
>  
> @@ -3591,8 +3584,12 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage, int nr)
>  			has_cache = SWAP_HAS_CACHE;
>  		else if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
>  			count += usage;
> -		else
> +		else if (swap_count_continued(p, offset + i, count))
>  			count = COUNT_CONTINUED;
> +		else {
> +			err = -ENOMEM;
> +			goto unlock_out;
> +		}
>  
>  		WRITE_ONCE(p->swap_map[offset + i], count | has_cache);
>  	}
>
> This makes the two iterations become:
>
> 	for (i = 0; i < nr; i++) {
> 		count = p->swap_map[offset + i];
>
> 		/*
> 		 * swapin_readahead() doesn't check if a swap entry is valid, so the
> 		 * swap entry could be SWAP_MAP_BAD. Check here with lock held.
> 		 */
> 		if (unlikely(swap_count(count) == SWAP_MAP_BAD)) {
> 			err = -ENOENT;
> 			goto unlock_out;
> 		}
>
> 		has_cache = count & SWAP_HAS_CACHE;
> 		count &= ~SWAP_HAS_CACHE;
>
> 		if (usage == SWAP_HAS_CACHE) {
> 			/* set SWAP_HAS_CACHE if there is no cache and entry is used */

The comments doen't apply now, we don't "set" here.

> 			if (!has_cache && count)
> 				continue;
> 			else if (has_cache)		/* someone else added cache */
> 				err = -EEXIST;
> 			else				/* no users remaining */
> 				err = -ENOENT;
> 		} else if (count || has_cache) {
> 			if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
> 				err = -EINVAL;
> 		} else
> 			err = -ENOENT;			/* unused swap entry */

It seems that this can be simplified to:

                if (!count && !has_cache) {
                        err = -ENOENT;
                } else if (usage == SWAP_HAS_CACHE) {
                        if (has_cache)
                                err = -EEXIST;
                } else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX) {
                        err = -EINVAL;
                }

> 		if (err)
> 			goto unlock_out;
> 	}
>
> 	for (i = 0; i < nr; i++) {
> 		count = p->swap_map[offset + i];
> 		has_cache = count & SWAP_HAS_CACHE;
> 		count &= ~SWAP_HAS_CACHE;
>
> 		if (usage == SWAP_HAS_CACHE)
> 			has_cache = SWAP_HAS_CACHE;
> 		else if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
> 			count += usage;
> 		else if (swap_count_continued(p, offset + i, count))
> 			count = COUNT_CONTINUED;
> 		else {

Better to add some comments here,

                        /*
                         * Don't need to rollback changes, because if
                         * usage == 1, there must be nr == 1.
                         */

> 			err = -ENOMEM;
> 			goto unlock_out;
> 		}
>
> 		WRITE_ONCE(p->swap_map[offset + i], count | has_cache);
> 	}
>
> Ying, do you feel more satisfied with the version above
> compared to the code in mm-unstable?

This looks good to me except some minor comments above.  Thanks!

--
Best Regards,
Huang, Ying

> Baolin, I'd also appreciate it if you could re-test your
> shmem case if Ying is okay with the enhancement above.
>
>>
>> --
>> Best Regards,
>> Huang, Ying
>>
>> >> >>
>> >> >> >> >>
>> >> >> >> >> > +                     else
>> >> >> >> >> > +                             err = -ENOMEM;
>> >> >> >> >> > +             } else
>> >> >> >> >> > +                     err = -ENOENT;                  /* unused swap entry */
>> >> >> >> >> >
>> >> >> >> >> > -             if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
>> >> >> >> >> > +             if (err)
>> >> >> >> >> > +                     goto unlock_out;
>> >> >> >> >> > +     }
>> >> >> >> >> > +
>> >> >> >> >> > +     for (i = 0; i < nr; i++) {
>> >> >> >> >> > +             count = p->swap_map[offset + i];
>> >> >> >> >> > +             has_cache = count & SWAP_HAS_CACHE;
>> >> >> >> >> > +             count &= ~SWAP_HAS_CACHE;
>> >> >> >> >> > +
>> >> >> >> >> > +             if (usage == SWAP_HAS_CACHE)
>> >> >> >> >> > +                     has_cache = SWAP_HAS_CACHE;
>> >> >> >> >> > +             else if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
>> >> >> >> >> >                       count += usage;
>> >> >> >> >> > -             else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
>> >> >> >> >> > -                     err = -EINVAL;
>> >> >> >> >> > -             else if (swap_count_continued(p, offset, count))
>> >> >> >> >> > -                     count = COUNT_CONTINUED;
>> >> >> >> >> >               else
>> >> >> >> >> > -                     err = -ENOMEM;
>> >> >> >> >> > -     } else
>> >> >> >> >> > -             err = -ENOENT;                  /* unused swap entry */
>> >> >> >> >> > +                     count = COUNT_CONTINUED;
>> >> >> >> >> >
>> >> >> >> >> > -     if (!err)
>> >> >> >> >> > -             WRITE_ONCE(p->swap_map[offset], count | has_cache);
>> >> >> >> >> > +             WRITE_ONCE(p->swap_map[offset + i], count | has_cache);
>> >> >> >> >> > +     }
>> >> >> >> >> >
>> >> >> >> >> >  unlock_out:
>> >> >> >> >> >       unlock_cluster_or_swap_info(p, ci);
>> >> >> >> >> > @@ -3439,7 +3456,7 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
>> >> >> >> >> >   */
>> >> >> >> >> >  void swap_shmem_alloc(swp_entry_t entry)
>> >> >> >> >> >  {
>> >> >> >> >> > -     __swap_duplicate(entry, SWAP_MAP_SHMEM);
>> >> >> >> >> > +     __swap_duplicate(entry, SWAP_MAP_SHMEM, 1);
>> >> >> >> >> >  }
>> >> >> >> >> >
>> >> >> >> >> >  /*
>> >> >> >> >> > @@ -3453,29 +3470,29 @@ int swap_duplicate(swp_entry_t entry)
>> >> >> >> >> >  {
>> >> >> >> >> >       int err = 0;
>> >> >> >> >> >
>> >> >> >> >> > -     while (!err && __swap_duplicate(entry, 1) == -ENOMEM)
>> >> >> >> >> > +     while (!err && __swap_duplicate(entry, 1, 1) == -ENOMEM)
>> >> >> >> >> >               err = add_swap_count_continuation(entry, GFP_ATOMIC);
>> >> >> >> >> >       return err;
>> >> >> >> >> >  }
>> >> >> >> >> >
>> >> >> >> >> >  /*
>> >> >> >> >> > - * @entry: swap entry for which we allocate swap cache.
>> >> >> >> >> > + * @entry: first swap entry from which we allocate nr swap cache.
>> >> >> >> >> >   *
>> >> >> >> >> > - * Called when allocating swap cache for existing swap entry,
>> >> >> >> >> > + * Called when allocating swap cache for existing swap entries,
>> >> >> >> >> >   * This can return error codes. Returns 0 at success.
>> >> >> >> >> >   * -EEXIST means there is a swap cache.
>> >> >> >> >> >   * Note: return code is different from swap_duplicate().
>> >> >> >> >> >   */
>> >> >> >> >> > -int swapcache_prepare(swp_entry_t entry)
>> >> >> >> >> > +int swapcache_prepare(swp_entry_t entry, int nr)
>> >> >> >> >> >  {
>> >> >> >> >> > -     return __swap_duplicate(entry, SWAP_HAS_CACHE);
>> >> >> >> >> > +     return __swap_duplicate(entry, SWAP_HAS_CACHE, nr);
>> >> >> >> >> >  }
>> >> >> >> >> >
>> >> >> >> >> > -void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry)
>> >> >> >> >> > +void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry, int nr)
>> >> >> >> >> >  {
>> >> >> >> >> >       unsigned long offset = swp_offset(entry);
>> >> >> >> >> >
>> >> >> >> >> > -     cluster_swap_free_nr(si, offset, 1, SWAP_HAS_CACHE);
>> >> >> >> >> > +     cluster_swap_free_nr(si, offset, nr, SWAP_HAS_CACHE);
>> >> >> >> >> >  }
>> >> >> >> >> >
>> >> >> >> >> >  struct swap_info_struct *swp_swap_info(swp_entry_t entry)
>> >> >> >> >>
>> >> >> >> >> --
>> >> >> >> >> Best Regards,
>> >> >> >> >> Huang, Ying
>> >> >> >> >
>> >> >
>
> Thanks
> Barry

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