[PATCH] mm: Cleanup - Reorganize the shrink_page_list code into smaller functions

From: Tim Chen
Date: Fri May 20 2016 - 17:33:22 EST

This patch consolidates the page out and the various cleanup operations
within shrink_page_list function into handle_pgout and pg_finish
functions.

This makes the shrink_page_list function more concise and allows for
the separation of page out and page scan operations.
It paves the way to group similar pages together and batch
process them in the page out path for better efficiency.

After we have scanned a page in shrink_page_list andÂ
completed paging, the final disposition and cleanÂ
up of the page is consolidated into pg_finish.ÂÂT
he designated disposition of the page from page scanning
in shrink_page_list is marked with one of the designation in pg_result.

There is no intention to change shrink_page_list's
functionality or logic in this patch.

Thanks.

Tim

Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
Âmm/vmscan.c | 429 ++++++++++++++++++++++++++++++++++--------------------------
Â1 file changed, 246 insertions(+), 183 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 142cb61..0eb3c67 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -873,6 +873,216 @@ static void page_check_dirty_writeback(struct page *page,
 mapping->a_ops->is_dirty_writeback(page, dirty, writeback);
Â}
Â
+enum pg_result {
+ PG_SPECULATIVE_REF,
+ PG_FREE,
+ PG_MLOCKED,
+ PG_ACTIVATE_LOCKED,
+ PG_KEEP_LOCKED,
+ PG_KEEP,
+ PG_NEXT,
+ PG_UNKNOWN,
+};
+
+static enum pg_result handle_pgout(struct list_head *page_list,
+ struct zone *zone,
+ struct scan_control *sc,
+ enum ttu_flags ttu_flags,
+ enum page_references references,
+ bool may_enter_fs,
+ bool lazyfree,
+ intÂÂ*swap_ret,
+ struct page *page)
+{
+ struct address_space *mapping;
+
+ mapping =ÂÂpage_mapping(page);
+
+ /*
+ Â* The page is mapped into the page tables of one or more
+ Â* processes. Try to unmap it here.
+ Â*/
+ if (page_mapped(page) && mapping) {
+ switch (*swap_ret = try_to_unmap(page, lazyfree ?
+ (ttu_flags | TTU_BATCH_FLUSH | TTU_LZFREE) :
+ (ttu_flags | TTU_BATCH_FLUSH))) {
+ case SWAP_FAIL:
+ return PG_ACTIVATE_LOCKED;
+ case SWAP_AGAIN:
+ return PG_KEEP_LOCKED;
+ case SWAP_MLOCK:
+ return PG_MLOCKED;
+ case SWAP_LZFREE:
+ goto lazyfree;
+ case SWAP_SUCCESS:
+ ; /* try to free the page below */
+ }
+ }
+
+ if (PageDirty(page)) {
+ /*
+ Â* Only kswapd can writeback filesystem pages to
+ Â* avoid risk of stack overflow but only writeback
+ Â* if many dirty pages have been encountered.
+ Â*/
+ if (page_is_file_cache(page) &&
+ (!current_is_kswapd() ||
+ Â!test_bit(ZONE_DIRTY, &zone->flags))) {
+ /*
+ Â* Immediately reclaim when written back.
+ Â* Similar in principal to deactivate_page()
+ Â* except we already have the page isolated
+ Â* and know it's dirty
+ Â*/
+ inc_zone_page_state(page, NR_VMSCAN_IMMEDIATE);
+ SetPageReclaim(page);
+
+ return PG_KEEP_LOCKED;
+ }
+
+ if (references == PAGEREF_RECLAIM_CLEAN)
+ return PG_KEEP_LOCKED;
+ if (!may_enter_fs)
+ return PG_KEEP_LOCKED;
+ if (!sc->may_writepage)
+ return PG_KEEP_LOCKED;
+
+ /*
+ Â* Page is dirty. Flush the TLB if a writable entry
+ Â* potentially exists to avoid CPU writes after IO
+ Â* starts and then write it out here.
+ Â*/
+ try_to_unmap_flush_dirty();
+ switch (pageout(page, mapping, sc)) {
+ case PAGE_KEEP:
+ return PG_KEEP_LOCKED;
+ case PAGE_ACTIVATE:
+ return PG_ACTIVATE_LOCKED;
+ case PAGE_SUCCESS:
+ if (PageWriteback(page))
+ return PG_KEEP;
+ if (PageDirty(page))
+ return PG_KEEP;
+
+ /*
+ Â* A synchronous write - probably a ramdisk.ÂÂGo
+ Â* ahead and try to reclaim the page.
+ Â*/
+ if (!trylock_page(page))
+ return PG_KEEP;
+ if (PageDirty(page) || PageWriteback(page))
+ return PG_KEEP_LOCKED;
+ mapping = page_mapping(page);
+ case PAGE_CLEAN:
+ ; /* try to free the page below */
+ }
+ }
+
+ /*
+ Â* If the page has buffers, try to free the buffer mappings
+ Â* associated with this page. If we succeed we try to free
+ Â* the page as well.
+ Â*
+ Â* We do this even if the page is PageDirty().
+ Â* try_to_release_page() does not perform I/O, but it is
+ Â* possible for a page to have PageDirty set, but it is actually
+ Â* clean (all its buffers are clean).ÂÂThis happens if the
+ Â* buffers were written out directly, with submit_bh(). ext3
+ Â* will do this, as well as the blockdev mapping.
+ Â* try_to_release_page() will discover that cleanness and will
+ Â* drop the buffers and mark the page clean - it can be freed.
+ Â*
+ Â* Rarely, pages can have buffers and no ->mapping.ÂÂThese are
+ Â* the pages which were not successfully invalidated in
+ Â* truncate_complete_page().ÂÂWe try to drop those buffers here
+ Â* and if that worked, and the page is no longer mapped into
+ Â* process address space (page_count == 1) it can be freed.
+ Â* Otherwise, leave the page on the LRU so it is swappable.
+ Â*/
+ if (page_has_private(page)) {
+ if (!try_to_release_page(page, sc->gfp_mask))
+ return PG_ACTIVATE_LOCKED;
+ if (!mapping && page_count(page) == 1) {
+ unlock_page(page);
+ if (put_page_testzero(page))
+ return PG_FREE;
+ else {
+ /*
+ Â* rare race with speculative reference.
+ Â* the speculative reference will free
+ Â* this page shortly, so we may
+ Â* increment nr_reclaimed (and
+ Â* leave it off the LRU).
+ Â*/
+ return PG_SPECULATIVE_REF;
+ }
+ }
+ }
+
+lazyfree:
+ if (!mapping || !__remove_mapping(mapping, page, true))
+ return PG_KEEP_LOCKED;
+
+ /*
+ Â* At this point, we have no other references and there is
+ Â* no way to pick any more up (removed from LRU, removed
+ Â* from pagecache). Can use non-atomic bitops now (and
+ Â* we obviously don't have to worry about waking up a process
+ Â* waiting on the page lock, because there are no references.
+ Â*/
+ __ClearPageLocked(page);
+ return PG_FREE;
+}
+
+static void pg_finish(struct page *page,
+ enum pg_result pg_dispose,
+ int swap_ret,
+ unsigned long *nr_reclaimed,
+ int *pgactivate,
+ struct list_head *ret_pages,
+ struct list_head *free_pages)
+{
+ switch (pg_dispose) {
+ case PG_SPECULATIVE_REF:
+ ++*nr_reclaimed;
+ return;
+ case PG_FREE:
+ if (swap_ret == SWAP_LZFREE)
+ count_vm_event(PGLAZYFREED);
+
+ ++*nr_reclaimed;
+ /*
+ Â* Is there need to periodically free_page_list? It would
+ Â* appear not as the counts should be low
+ Â*/
+ list_add(&page->lru, free_pages);
+ return;
+ case PG_MLOCKED:
+ if (PageSwapCache(page))
+ try_to_free_swap(page);
+ unlock_page(page);
+ list_add(&page->lru, ret_pages);
+ return;
+ case PG_ACTIVATE_LOCKED:
+ /* Not a candidate for swapping, so reclaim swap space. */
+ if (PageSwapCache(page) && mem_cgroup_swap_full(page))
+ try_to_free_swap(page);
+ VM_BUG_ON_PAGE(PageActive(page), page);
+ SetPageActive(page);
+ ++*pgactivate;
+ case PG_KEEP_LOCKED:
+ unlock_page(page);
+ case PG_KEEP:
+ list_add(&page->lru, ret_pages);
+ case PG_NEXT:
+ VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
+ return;
+ case PG_UNKNOWN:
+ VM_BUG_ON_PAGE((pg_dispose == PG_UNKNOWN), page);
+ return;
+ }
+}
+
Â/*
 * shrink_page_list() returns the number of reclaimed pages
 */
@@ -904,28 +1114,35 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 struct page *page;
 int may_enter_fs;
 enum page_references references = PAGEREF_RECLAIM_CLEAN;
+ enum pg_result pg_dispose = PG_UNKNOWN;
 bool dirty, writeback;
 bool lazyfree = false;
- int ret = SWAP_SUCCESS;
+ int swap_ret = SWAP_SUCCESS;
Â
 cond_resched();
Â
 page = lru_to_page(page_list);
 list_del(&page->lru);
Â
- if (!trylock_page(page))
- goto keep;
+ if (!trylock_page(page)) {
+ pg_dispose = PG_KEEP;
+ goto finish;
+ }
Â
 VM_BUG_ON_PAGE(PageActive(page), page);
 VM_BUG_ON_PAGE(page_zone(page) != zone, page);
Â
 sc->nr_scanned++;
Â
- if (unlikely(!page_evictable(page)))
- goto cull_mlocked;
+ if (unlikely(!page_evictable(page))) {
+ pg_dispose = PG_MLOCKED;
+ goto finish;
+ }
Â
- if (!sc->may_unmap && page_mapped(page))
- goto keep_locked;
+ if (!sc->may_unmap && page_mapped(page)) {
+ pg_dispose = PG_KEEP_LOCKED;
+ goto finish;
+ }
Â
 /* Double the slab pressure for mapped and swapcache pages */
 if (page_mapped(page) || PageSwapCache(page))
@@ -998,7 +1215,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 ÂÂÂÂPageReclaim(page) &&
 ÂÂÂÂtest_bit(ZONE_WRITEBACK, &zone->flags)) {
 nr_immediate++;
- goto keep_locked;
+ pg_dispose = PG_KEEP_LOCKED;
+ goto finish;
Â
 /* Case 2 above */
 } else if (sane_reclaim(sc) ||
@@ -1016,7 +1234,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 Â*/
 SetPageReclaim(page);
 nr_writeback++;
- goto keep_locked;
+ pg_dispose = PG_KEEP_LOCKED;
+ goto finish;
Â
 /* Case 3 above */
 } else {
@@ -1033,9 +1252,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
Â
 switch (references) {
 case PAGEREF_ACTIVATE:
- goto activate_locked;
+ pg_dispose = PG_ACTIVATE_LOCKED;
+ goto finish;
 case PAGEREF_KEEP:
- goto keep_locked;
+ pg_dispose = PG_KEEP_LOCKED;
+ goto finish;
 case PAGEREF_RECLAIM:
 case PAGEREF_RECLAIM_CLEAN:
 ; /* try to reclaim the page below */
@@ -1046,183 +1267,25 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 Â* Try to allocate it some swap space here.
 Â*/
 if (PageAnon(page) && !PageSwapCache(page)) {
- if (!(sc->gfp_mask & __GFP_IO))
- goto keep_locked;
- if (!add_to_swap(page, page_list))
- goto activate_locked;
- lazyfree = true;
- may_enter_fs = 1;
-
- /* Adding to swap updated mapping */
- mapping = page_mapping(page);
- }
-
- /*
- Â* The page is mapped into the page tables of one or more
- Â* processes. Try to unmap it here.
- Â*/
- if (page_mapped(page) && mapping) {
- switch (ret = try_to_unmap(page, lazyfree ?
- (ttu_flags | TTU_BATCH_FLUSH | TTU_LZFREE) :
- (ttu_flags | TTU_BATCH_FLUSH))) {
- case SWAP_FAIL:
- goto activate_locked;
- case SWAP_AGAIN:
- goto keep_locked;
- case SWAP_MLOCK:
- goto cull_mlocked;
- case SWAP_LZFREE:
- goto lazyfree;
- case SWAP_SUCCESS:
- ; /* try to free the page below */
+ if (!(sc->gfp_mask & __GFP_IO)) {
+ pg_dispose = PG_KEEP_LOCKED;
+ goto finish;
 }
- }
-
- if (PageDirty(page)) {
- /*
- Â* Only kswapd can writeback filesystem pages to
- Â* avoid risk of stack overflow but only writeback
- Â* if many dirty pages have been encountered.
- Â*/
- if (page_is_file_cache(page) &&
- (!current_is_kswapd() ||
- Â!test_bit(ZONE_DIRTY, &zone->flags))) {
- /*
- Â* Immediately reclaim when written back.
- Â* Similar in principal to deactivate_page()
- Â* except we already have the page isolated
- Â* and know it's dirty
- Â*/
- inc_zone_page_state(page, NR_VMSCAN_IMMEDIATE);
- SetPageReclaim(page);
-
- goto keep_locked;
- }
-
- if (references == PAGEREF_RECLAIM_CLEAN)
- goto keep_locked;
- if (!may_enter_fs)
- goto keep_locked;
- if (!sc->may_writepage)
- goto keep_locked;
-
- /*
- Â* Page is dirty. Flush the TLB if a writable entry
- Â* potentially exists to avoid CPU writes after IO
- Â* starts and then write it out here.
- Â*/
- try_to_unmap_flush_dirty();
- switch (pageout(page, mapping, sc)) {
- case PAGE_KEEP:
- goto keep_locked;
- case PAGE_ACTIVATE:
- goto activate_locked;
- case PAGE_SUCCESS:
- if (PageWriteback(page))
- goto keep;
- if (PageDirty(page))
- goto keep;
-
- /*
- Â* A synchronous write - probably a ramdisk.ÂÂGo
- Â* ahead and try to reclaim the page.
- Â*/
- if (!trylock_page(page))
- goto keep;
- if (PageDirty(page) || PageWriteback(page))
- goto keep_locked;
- mapping = page_mapping(page);
- case PAGE_CLEAN:
- ; /* try to free the page below */
- }
- }
-
- /*
- Â* If the page has buffers, try to free the buffer mappings
- Â* associated with this page. If we succeed we try to free
- Â* the page as well.
- Â*
- Â* We do this even if the page is PageDirty().
- Â* try_to_release_page() does not perform I/O, but it is
- Â* possible for a page to have PageDirty set, but it is actually
- Â* clean (all its buffers are clean).ÂÂThis happens if the
- Â* buffers were written out directly, with submit_bh(). ext3
- Â* will do this, as well as the blockdev mapping.
- Â* try_to_release_page() will discover that cleanness and will
- Â* drop the buffers and mark the page clean - it can be freed.
- Â*
- Â* Rarely, pages can have buffers and no ->mapping.ÂÂThese are
- Â* the pages which were not successfully invalidated in
- Â* truncate_complete_page().ÂÂWe try to drop those buffers here
- Â* and if that worked, and the page is no longer mapped into
- Â* process address space (page_count == 1) it can be freed.
- Â* Otherwise, leave the page on the LRU so it is swappable.
- Â*/
- if (page_has_private(page)) {
- if (!try_to_release_page(page, sc->gfp_mask))
- goto activate_locked;
- if (!mapping && page_count(page) == 1) {
- unlock_page(page);
- if (put_page_testzero(page))
- goto free_it;
- else {
- /*
- Â* rare race with speculative reference.
- Â* the speculative reference will free
- Â* this page shortly, so we may
- Â* increment nr_reclaimed here (and
- Â* leave it off the LRU).
- Â*/
- nr_reclaimed++;
- continue;
- }
+ if (!add_to_swap(page, page_list)) {
+ pg_dispose = PG_ACTIVATE_LOCKED;
+ goto finish;
 }
+ lazyfree = true;
+ may_enter_fs = 1;
 }
Â
-lazyfree:
- if (!mapping || !__remove_mapping(mapping, page, true))
- goto keep_locked;
-
- /*
- Â* At this point, we have no other references and there is
- Â* no way to pick any more up (removed from LRU, removed
- Â* from pagecache). Can use non-atomic bitops now (and
- Â* we obviously don't have to worry about waking up a process
- Â* waiting on the page lock, because there are no references.
- Â*/
- __ClearPageLocked(page);
-free_it:
- if (ret == SWAP_LZFREE)
- count_vm_event(PGLAZYFREED);
-
- nr_reclaimed++;
+ pg_dispose = handle_pgout(page_list, zone, sc, ttu_flags,
+ references, may_enter_fs, lazyfree,
+ &swap_ret, page);
+finish:
+ pg_finish(page, pg_dispose, swap_ret, &nr_reclaimed,
+ &pgactivate, &ret_pages, &free_pages);
Â
- /*
- Â* Is there need to periodically free_page_list? It would
- Â* appear not as the counts should be low
- Â*/
- list_add(&page->lru, &free_pages);
- continue;
-
-cull_mlocked:
- if (PageSwapCache(page))
- try_to_free_swap(page);
- unlock_page(page);
- list_add(&page->lru, &ret_pages);
- continue;
-
-activate_locked:
- /* Not a candidate for swapping, so reclaim swap space. */
- if (PageSwapCache(page) && mem_cgroup_swap_full(page))
- try_to_free_swap(page);
- VM_BUG_ON_PAGE(PageActive(page), page);
- SetPageActive(page);
- pgactivate++;
-keep_locked:
- unlock_page(page);
-keep:
- list_add(&page->lru, &ret_pages);
- VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
 }
Â
 mem_cgroup_uncharge_list(&free_pages);
--Â
2.5.5