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

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

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);
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