Re: [PATCH v2 4/4] mm:swap: skip swapcache for swapin of synchronous device

["Huang\, Ying" <ying.huang@intel.com>]

Minchan Kim <minchan@kernel.org> writes:

> Hi Huang,
>
> Sorry for the late response. It was long national holiday.
>
> On Fri, Sep 29, 2017 at 04:51:17PM +0800, huang ying wrote:
>> On Wed, Sep 20, 2017 at 1:43 PM, Minchan Kim <minchan@kernel.org> wrote:
>> > With fast swap storage, platform want to use swap more aggressively
>> > and swap-in is crucial to application latency.
>> >
>> > The rw_page based synchronous devices like zram, pmem and btt are such
>> > fast storage. When I profile swapin performance with zram lz4 decompress
>> > test, S/W overhead is more than 70%. Maybe, it would be bigger in nvdimm.
>> >
>> > This patch aims for reducing swap-in latency via skipping swapcache
>> > if swap device is synchronous device like rw_page based device.
>> > It enhances 45% my swapin test(5G sequential swapin, no readahead,
>> > from 2.41sec to 1.64sec).
>> >
>> > Cc: Dan Williams <dan.j.williams@intel.com>
>> > Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
>> > Cc: Hugh Dickins <hughd@google.com>
>> > Signed-off-by: Minchan Kim <minchan@kernel.org>
>> > ---
>> >  include/linux/swap.h | 11 +++++++++++
>> >  mm/memory.c          | 52 ++++++++++++++++++++++++++++++++++++----------------
>> >  mm/page_io.c         |  6 +++---
>> >  mm/swapfile.c        | 11 +++++++----
>> >  4 files changed, 57 insertions(+), 23 deletions(-)
>> >
>> > diff --git a/include/linux/swap.h b/include/linux/swap.h
>> > index fbb33919d1c6..cd2f66fdfc2d 100644
>> > --- a/include/linux/swap.h
>> > +++ b/include/linux/swap.h
>> > @@ -461,6 +461,7 @@ extern int page_swapcount(struct page *);
>> >  extern int __swp_swapcount(swp_entry_t entry);
>> >  extern int swp_swapcount(swp_entry_t entry);
>> >  extern struct swap_info_struct *page_swap_info(struct page *);
>> > +extern struct swap_info_struct *swp_swap_info(swp_entry_t entry);
>> >  extern bool reuse_swap_page(struct page *, int *);
>> >  extern int try_to_free_swap(struct page *);
>> >  struct backing_dev_info;
>> > @@ -469,6 +470,16 @@ extern void exit_swap_address_space(unsigned int type);
>> >
>> >  #else /* CONFIG_SWAP */
>> >
>> > +static inline int swap_readpage(struct page *page, bool do_poll)
>> > +{
>> > +       return 0;
>> > +}
>> > +
>> > +static inline struct swap_info_struct *swp_swap_info(swp_entry_t entry)
>> > +{
>> > +       return NULL;
>> > +}
>> > +
>> >  #define swap_address_space(entry)              (NULL)
>> >  #define get_nr_swap_pages()                    0L
>> >  #define total_swap_pages                       0L
>> > diff --git a/mm/memory.c b/mm/memory.c
>> > index ec4e15494901..163ab2062385 100644
>> > --- a/mm/memory.c
>> > +++ b/mm/memory.c
>> > @@ -2842,7 +2842,7 @@ EXPORT_SYMBOL(unmap_mapping_range);
>> >  int do_swap_page(struct vm_fault *vmf)
>> >  {
>> >         struct vm_area_struct *vma = vmf->vma;
>> > -       struct page *page = NULL, *swapcache;
>> > +       struct page *page = NULL, *swapcache = NULL;
>> >         struct mem_cgroup *memcg;
>> >         struct vma_swap_readahead swap_ra;
>> >         swp_entry_t entry;
>> > @@ -2881,17 +2881,35 @@ int do_swap_page(struct vm_fault *vmf)
>> >                 }
>> >                 goto out;
>> >         }
>> > +
>> > +
>> >         delayacct_set_flag(DELAYACCT_PF_SWAPIN);
>> >         if (!page)
>> >                 page = lookup_swap_cache(entry, vma_readahead ? vma : NULL,
>> >                                          vmf->address);
>> >         if (!page) {
>> > -               if (vma_readahead)
>> > -                       page = do_swap_page_readahead(entry,
>> > -                               GFP_HIGHUSER_MOVABLE, vmf, &swap_ra);
>> > -               else
>> > -                       page = swapin_readahead(entry,
>> > -                               GFP_HIGHUSER_MOVABLE, vma, vmf->address);
>> > +               struct swap_info_struct *si = swp_swap_info(entry);
>> > +
>> > +               if (!(si->flags & SWP_SYNCHRONOUS_IO)) {
>> > +                       if (vma_readahead)
>> > +                               page = do_swap_page_readahead(entry,
>> > +                                       GFP_HIGHUSER_MOVABLE, vmf, &swap_ra);
>> > +                       else
>> > +                               page = swapin_readahead(entry,
>> > +                                       GFP_HIGHUSER_MOVABLE, vma, vmf->address);
>> > +                       swapcache = page;
>> > +               } else {
>> > +                       /* skip swapcache */
>> > +                       page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, vmf->address);
>> > +                       if (page) {
>> > +                               __SetPageLocked(page);
>> > +                               __SetPageSwapBacked(page);
>> > +                               set_page_private(page, entry.val);
>> > +                               lru_cache_add_anon(page);
>> > +                               swap_readpage(page, true);
>> > +                       }
>> > +               }
>> 
>> I have a question for this.  If a page is mapped in multiple processes
>> (for example, because of fork).  With swap cache, after swapping out
>> and swapping in, the page will be still shared by these processes.
>> But with your changes, it appears that there will be multiple pages
>> with same contents mapped in multiple processes, even if the page
>> isn't written in these processes.  So this may waste some memory in
>> some situation?  And copying from device is even faster than looking
>> up swap cache in your system?
>
> I expected a page shared by several processes has low possibility to swap out
> compared to a single mapped page. Nonetheless, once it is swapped out, it also
> has low chance to swap in so I didn't cover the case intentionally until we
> get any regression report.

Thanks for explanation.

> However, a fix would be simple so I don't care to add up it.
> Any thoughts?

I think the fix can work well with shared anonymous pages in most cases
(although not all).  It should be good to add it.

Best Regards,
Huang, Ying

> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index cd2f66fdfc2d..23f19ffa5cc3 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -458,6 +458,7 @@ extern unsigned int count_swap_pages(int, int);
>  extern sector_t map_swap_page(struct page *, struct block_device **);
>  extern sector_t swapdev_block(int, pgoff_t);
>  extern int page_swapcount(struct page *);
> +extern int __swap_count(struct swap_info_struct *si, swp_entry_t entry);
>  extern int __swp_swapcount(swp_entry_t entry);
>  extern int swp_swapcount(swp_entry_t entry);
>  extern struct swap_info_struct *page_swap_info(struct page *);
> @@ -584,6 +585,11 @@ static inline int page_swapcount(struct page *page)
>  	return 0;
>  }
>  
> +static inline int __swap_count(structd swap_info_struct *si, swp_entry_t entry)
> +{
> +	return 0;
> +}
> +
>  static inline int __swp_swapcount(swp_entry_t entry)
>  {
>  	return 0;
> diff --git a/include/linux/swapfile.h b/include/linux/swapfile.h
> index 388293a91e8c..49f8e19dd506 100644
> --- a/include/linux/swapfile.h
> +++ b/include/linux/swapfile.h
> @@ -9,5 +9,4 @@ extern spinlock_t swap_lock;
>  extern struct plist_head swap_active_head;
>  extern struct swap_info_struct *swap_info[];
>  extern int try_to_unuse(unsigned int, bool, unsigned long);
> -
>  #endif /* _LINUX_SWAPFILE_H */
> diff --git a/mm/memory.c b/mm/memory.c
> index 163ab2062385..c6f0abe8b39b 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2890,15 +2890,8 @@ int do_swap_page(struct vm_fault *vmf)
>  	if (!page) {
>  		struct swap_info_struct *si = swp_swap_info(entry);
>  
> -		if (!(si->flags & SWP_SYNCHRONOUS_IO)) {
> -			if (vma_readahead)
> -				page = do_swap_page_readahead(entry,
> -					GFP_HIGHUSER_MOVABLE, vmf, &swap_ra);
> -			else
> -				page = swapin_readahead(entry,
> -					GFP_HIGHUSER_MOVABLE, vma, vmf->address);
> -			swapcache = page;
> -		} else {
> +		if ((si->flags & SWP_SYNCHRONOUS_IO) && (vmf->flags & FAULT_FLAG_WRITE ||
> +							__swap_count(si, entry) == 1)) {
>  			/* skip swapcache */
>  			page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, vmf->address);
>  			if (page) {
> @@ -2908,6 +2901,14 @@ int do_swap_page(struct vm_fault *vmf)
>  				lru_cache_add_anon(page);
>  				swap_readpage(page, true);
>  			}
> +		} else {
> +			if (vma_readahead)
> +				page = do_swap_page_readahead(entry,
> +					GFP_HIGHUSER_MOVABLE, vmf, &swap_ra);
> +			else
> +				page = swapin_readahead(entry,
> +					GFP_HIGHUSER_MOVABLE, vma, vmf->address);
> +			swapcache = page;
>  		}
>  
>  		if (!page) {
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 64a3d85226ba..37d7ba71a2ca 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1328,7 +1328,13 @@ int page_swapcount(struct page *page)
>  	return count;
>  }
>  
> -static int swap_swapcount(struct swap_info_struct *si, swp_entry_t entry)
> +int __swap_count(struct swap_info_struct *si, swp_entry_t entry)
> +{
> +	pgoff_t offset = swp_offset(entry);
> +	return swap_count(si->swap_map[offset]);
> +}
> +
> +int swap_swapcount(struct swap_info_struct *si, swp_entry_t entry)
>  {
>  	int count = 0;
>  	pgoff_t offset = swp_offset(entry);