[PATCH 5/5] mm:swap: skip swapcache for swapin of synchronous device

From: Minchan Kim
Date: Mon Sep 11 2017 - 22:38:09 EST

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@xxxxxxxxx>
Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
include/linux/swap.h | 1 +
mm/memory.c | 48 +++++++++++++++++++++++++++++++++---------------
mm/page_io.c | 6 +++---
mm/swapfile.c | 11 +++++++----
4 files changed, 44 insertions(+), 22 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 739d94397c47..fb46a3b55d65 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -462,6 +462,7 @@ 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;
diff --git a/mm/memory.c b/mm/memory.c
index ec4e15494901..130698cd2d98 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 +2881,33 @@ 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);
+ __SetPageLocked(page);
+ __SetPageSwapBacked(page);
+ set_page_private(page, entry.val);
+ lru_cache_add_anon(page);
+ swap_readpage(page, true);
+ }
+
if (!page) {
/*
* Back out if somebody else faulted in this pte
@@ -2920,7 +2936,6 @@ int do_swap_page(struct vm_fault *vmf)
goto out_release;
}

- swapcache = page;
locked = lock_page_or_retry(page, vma->vm_mm, vmf->flags);

delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
@@ -2935,7 +2950,8 @@ int do_swap_page(struct vm_fault *vmf)
* test below, are not enough to exclude that. Even if it is still
* swapcache, we need to check that the page's swap has not changed.
*/
- if (unlikely(!PageSwapCache(page) || page_private(page) != entry.val))
+ if (unlikely((!PageSwapCache(page) ||
+ page_private(page) != entry.val)) && swapcache)
goto out_page;

page = ksm_might_need_to_copy(page, vma, vmf->address);
@@ -2988,14 +3004,16 @@ int do_swap_page(struct vm_fault *vmf)
pte = pte_mksoft_dirty(pte);
set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
vmf->orig_pte = pte;
- if (page == swapcache) {
- do_page_add_anon_rmap(page, vma, vmf->address, exclusive);
- mem_cgroup_commit_charge(page, memcg, true, false);
- activate_page(page);
- } else { /* ksm created a completely new copy */
+
+ /* ksm created a completely new copy */
+ if (unlikely(page != swapcache && swapcache)) {
page_add_new_anon_rmap(page, vma, vmf->address, false);
mem_cgroup_commit_charge(page, memcg, false, false);
lru_cache_add_active_or_unevictable(page, vma);
+ } else {
+ do_page_add_anon_rmap(page, vma, vmf->address, exclusive);
+ mem_cgroup_commit_charge(page, memcg, true, false);
+ activate_page(page);
}

swap_free(entry);
@@ -3003,7 +3021,7 @@ int do_swap_page(struct vm_fault *vmf)
(vma->vm_flags & VM_LOCKED) || PageMlocked(page))
try_to_free_swap(page);
unlock_page(page);
- if (page != swapcache) {
+ if (page != swapcache && swapcache) {
/*
* Hold the lock to avoid the swap entry to be reused
* until we take the PT lock for the pte_same() check
diff --git a/mm/page_io.c b/mm/page_io.c
index 21502d341a67..d4a98e1f6608 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -346,7 +346,7 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
return ret;
}

-int swap_readpage(struct page *page, bool do_poll)
+int swap_readpage(struct page *page, bool synchronous)
{
struct bio *bio;
int ret = 0;
@@ -354,7 +354,7 @@ int swap_readpage(struct page *page, bool do_poll)
blk_qc_t qc;
struct gendisk *disk;

- VM_BUG_ON_PAGE(!PageSwapCache(page), page);
+ VM_BUG_ON_PAGE(!PageSwapCache(page) && !synchronous, page);
VM_BUG_ON_PAGE(!PageLocked(page), page);
VM_BUG_ON_PAGE(PageUptodate(page), page);
if (frontswap_load(page) == 0) {
@@ -402,7 +402,7 @@ int swap_readpage(struct page *page, bool do_poll)
count_vm_event(PSWPIN);
bio_get(bio);
qc = submit_bio(bio);
- while (do_poll) {
+ while (synchronous) {
set_current_state(TASK_UNINTERRUPTIBLE);
if (!READ_ONCE(bio->bi_private))
break;
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 1305591cde4d..64a3d85226ba 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3454,10 +3454,15 @@ int swapcache_prepare(swp_entry_t entry)
return __swap_duplicate(entry, SWAP_HAS_CACHE);
}

+struct swap_info_struct *swp_swap_info(swp_entry_t entry)
+{
+ return swap_info[swp_type(entry)];
+}
+
struct swap_info_struct *page_swap_info(struct page *page)
{
- swp_entry_t swap = { .val = page_private(page) };
- return swap_info[swp_type(swap)];
+ swp_entry_t entry = { .val = page_private(page) };
+ return swp_swap_info(entry);
}

/*
@@ -3465,7 +3470,6 @@ struct swap_info_struct *page_swap_info(struct page *page)
*/
struct address_space *__page_file_mapping(struct page *page)
{
- VM_BUG_ON_PAGE(!PageSwapCache(page), page);
return page_swap_info(page)->swap_file->f_mapping;
}
EXPORT_SYMBOL_GPL(__page_file_mapping);
@@ -3473,7 +3477,6 @@ EXPORT_SYMBOL_GPL(__page_file_mapping);
pgoff_t __page_file_index(struct page *page)
{
swp_entry_t swap = { .val = page_private(page) };
- VM_BUG_ON_PAGE(!PageSwapCache(page), page);
return swp_offset(swap);
}
EXPORT_SYMBOL_GPL(__page_file_index);
--
2.7.4