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/*
* shrink_page_list() returns the number of reclaimed pages
*/
static unsigned int shrink_page_list(struct list_head *page_list,
struct pglist_data *pgdat,
struct scan_control *sc,
struct reclaim_stat *stat,
bool ignore_references)
{
LIST_HEAD(ret_pages);
LIST_HEAD(free_pages);
LIST_HEAD(demote_pages);
unsigned int nr_reclaimed = 0;
unsigned int pgactivate = 0;
bool do_demote_pass;
memset(stat, 0, sizeof(*stat));
cond_resched();
do_demote_pass = can_demote(pgdat->node_id, sc);
retry:
while (!list_empty(page_list)) {
struct address_space *mapping;
struct page *page;
enum page_references references = PAGEREF_RECLAIM;
bool dirty, writeback, may_enter_fs;
unsigned int nr_pages;
cond_resched();
page = lru_to_page(page_list);
list_del(&page->lru);
if (!trylock_page(page))
goto keep;
VM_BUG_ON_PAGE(PageActive(page), page);
nr_pages = compound_nr(page);
/* Account the number of base pages even though THP */
sc->nr_scanned += nr_pages;
if (unlikely(!page_evictable(page)))
goto activate_locked;
if (!sc->may_unmap && page_mapped(page))
goto keep_locked;
may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
/*
* The number of dirty pages determines if a node is marked
* reclaim_congested which affects wait_iff_congested. kswapd
* will stall and start writing pages if the tail of the LRU
* is all dirty unqueued pages.
*/
page_check_dirty_writeback(page, &dirty, &writeback);
if (dirty || writeback)
stat->nr_dirty++;
if (dirty && !writeback)
stat->nr_unqueued_dirty++;
/*
* Treat this page as congested if the underlying BDI is or if
* pages are cycling through the LRU so quickly that the
* pages marked for immediate reclaim are making it to the
* end of the LRU a second time.
*/
mapping = page_mapping(page);
if (((dirty || writeback) && mapping &&
inode_write_congested(mapping->host)) ||
(writeback && PageReclaim(page)))
stat->nr_congested++;
/*
* If a page at the tail of the LRU is under writeback, there
* are three cases to consider.
*
* 1) If reclaim is encountering an excessive number of pages
* under writeback and this page is both under writeback and
* PageReclaim then it indicates that pages are being queued
* for IO but are being recycled through the LRU before the
* IO can complete. Waiting on the page itself risks an
* indefinite stall if it is impossible to writeback the
* page due to IO error or disconnected storage so instead
* note that the LRU is being scanned too quickly and the
* caller can stall after page list has been processed.
*
* 2) Global or new memcg reclaim encounters a page that is
* not marked for immediate reclaim, or the caller does not
* have __GFP_FS (or __GFP_IO if it's simply going to swap,
* not to fs). In this case mark the page for immediate
* reclaim and continue scanning.
*
* Require may_enter_fs because we would wait on fs, which
* may not have submitted IO yet. And the loop driver might
* enter reclaim, and deadlock if it waits on a page for
* which it is needed to do the write (loop masks off
* __GFP_IO|__GFP_FS for this reason); but more thought
* would probably show more reasons.
*
* 3) Legacy memcg encounters a page that is already marked
* PageReclaim. memcg does not have any dirty pages
* throttling so we could easily OOM just because too many
* pages are in writeback and there is nothing else to
* reclaim. Wait for the writeback to complete.
*
* In cases 1) and 2) we activate the pages to get them out of
* the way while we continue scanning for clean pages on the
* inactive list and refilling from the active list. The
* observation here is that waiting for disk writes is more
* expensive than potentially causing reloads down the line.
* Since they're marked for immediate reclaim, they won't put
* memory pressure on the cache working set any longer than it
* takes to write them to disk.
*/
if (PageWriteback(page)) {
/* Case 1 above */
if (current_is_kswapd() &&
PageReclaim(page) &&
test_bit(PGDAT_WRITEBACK, &pgdat->flags)) {
stat->nr_immediate++;
goto activate_locked;
/* Case 2 above */
} else if (writeback_throttling_sane(sc) ||
!PageReclaim(page) || !may_enter_fs) {
/*
* This is slightly racy - end_page_writeback()
* might have just cleared PageReclaim, then
* setting PageReclaim here end up interpreted
* as PageReadahead - but that does not matter
* enough to care. What we do want is for this
* page to have PageReclaim set next time memcg
* reclaim reaches the tests above, so it will
* then wait_on_page_writeback() to avoid OOM;
* and it's also appropriate in global reclaim.
*/
SetPageReclaim(page);
stat->nr_writeback++;
goto activate_locked;
/* Case 3 above */
} else {
unlock_page(page);
wait_on_page_writeback(page);
/* then go back and try same page again */
list_add_tail(&page->lru, page_list);
continue;
}
}
if (!ignore_references)
references = page_check_references(page, sc);
switch (references) {
case PAGEREF_ACTIVATE:
goto activate_locked;
case PAGEREF_KEEP:
stat->nr_ref_keep += nr_pages;
goto keep_locked;
case PAGEREF_RECLAIM:
case PAGEREF_RECLAIM_CLEAN:
; /* try to reclaim the page below */
}
/*
* Before reclaiming the page, try to relocate
* its contents to another node.
*/
if (do_demote_pass &&
(thp_migration_supported() || !PageTransHuge(page))) {
list_add(&page->lru, &demote_pages);
unlock_page(page);
continue;
}
/*
* Anonymous process memory has backing store?
* Try to allocate it some swap space here.
* Lazyfree page could be freed directly
*/
if (PageAnon(page) && PageSwapBacked(page)) {
if (!PageSwapCache(page)) {
if (!(sc->gfp_mask & __GFP_IO))
goto keep_locked;
if (page_maybe_dma_pinned(page))
goto keep_locked;
if (PageTransHuge(page)) {
/* cannot split THP, skip it */
if (!can_split_huge_page(page, NULL))
goto activate_locked;
/*
* Split pages without a PMD map right
* away. Chances are some or all of the
* tail pages can be freed without IO.
*/
if (!compound_mapcount(page) &&
split_huge_page_to_list(page,
page_list))
goto activate_locked;
}
if (!add_to_swap(page)) {
if (!PageTransHuge(page))
goto activate_locked_split;
/* Fallback to swap normal pages */
if (split_huge_page_to_list(page,
page_list))
goto activate_locked;
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
count_vm_event(THP_SWPOUT_FALLBACK);
#endif
if (!add_to_swap(page))
goto activate_locked_split;
}
may_enter_fs = true;
/* Adding to swap updated mapping */
mapping = page_mapping(page);
}
} else if (unlikely(PageTransHuge(page))) {
/* Split file THP */
if (split_huge_page_to_list(page, page_list))
goto keep_locked;
}
/*
* THP may get split above, need minus tail pages and update
* nr_pages to avoid accounting tail pages twice.
*
* The tail pages that are added into swap cache successfully
* reach here.
*/
if ((nr_pages > 1) && !PageTransHuge(page)) {
sc->nr_scanned -= (nr_pages - 1);
nr_pages = 1;
}
/*
* The page is mapped into the page tables of one or more
* processes. Try to unmap it here.
*/
if (page_mapped(page)) {
enum ttu_flags flags = TTU_BATCH_FLUSH;
bool was_swapbacked = PageSwapBacked(page);
if (unlikely(PageTransHuge(page)))
flags |= TTU_SPLIT_HUGE_PMD;
try_to_unmap(page, flags);
if (page_mapped(page)) {
stat->nr_unmap_fail += nr_pages;
if (!was_swapbacked && PageSwapBacked(page))
stat->nr_lazyfree_fail += nr_pages;
goto activate_locked;
}
}
if (PageDirty(page)) {
/*
* Only kswapd can writeback filesystem pages
* to avoid risk of stack overflow. But avoid
* injecting inefficient single-page IO into
* flusher writeback as much as possible: only
* write pages when we've encountered many
* dirty pages, and when we've already scanned
* the rest of the LRU for clean pages and see
* the same dirty pages again (PageReclaim).
*/
if (page_is_file_lru(page) &&
(!current_is_kswapd() || !PageReclaim(page) ||
!test_bit(PGDAT_DIRTY, &pgdat->flags))) {
/*
* Immediately reclaim when written back.
* Similar in principal to deactivate_page()
* except we already have the page isolated
* and know it's dirty
*/
inc_node_page_state(page, NR_VMSCAN_IMMEDIATE);
SetPageReclaim(page);
goto activate_locked;
}
if (references == PAGEREF_RECLAIM_CLEAN)
goto keep_locked;
if (!may_enter_fs)
goto keep_locked;
if (!sc->may_writepage)
goto keep_locked;
/*
* Page is dirty. Flush the TLB if a writable entry
* potentially exists to avoid CPU writes after IO
* starts and then write it out here.
*/
try_to_unmap_flush_dirty();
switch (pageout(page, mapping)) {
case PAGE_KEEP:
goto keep_locked;
case PAGE_ACTIVATE:
goto activate_locked;
case PAGE_SUCCESS:
stat->nr_pageout += thp_nr_pages(page);
if (PageWriteback(page))
goto keep;
if (PageDirty(page))
goto keep;
/*
* A synchronous write - probably a ramdisk. Go
* ahead and try to reclaim the page.
*/
if (!trylock_page(page))
goto keep;
if (PageDirty(page) || PageWriteback(page))
goto keep_locked;
mapping = page_mapping(page);
fallthrough;
case PAGE_CLEAN:
; /* try to free the page below */
}
}
/*
* If the page has buffers, try to free the buffer mappings
* associated with this page. If we succeed we try to free
* the page as well.
*
* We do this even if the page is PageDirty().
* try_to_release_page() does not perform I/O, but it is
* possible for a page to have PageDirty set, but it is actually
* clean (all its buffers are clean). This happens if the
* buffers were written out directly, with submit_bh(). ext3
* will do this, as well as the blockdev mapping.
* try_to_release_page() will discover that cleanness and will
* drop the buffers and mark the page clean - it can be freed.
*
* Rarely, pages can have buffers and no ->mapping. These are
* the pages which were not successfully invalidated in
* truncate_cleanup_page(). We try to drop those buffers here
* and if that worked, and the page is no longer mapped into
* process address space (page_count == 1) it can be freed.
* Otherwise, leave the page on the LRU so it is swappable.
*/
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 {
/*
* rare race with speculative reference.
* the speculative reference will free
* this page shortly, so we may
* increment nr_reclaimed here (and
* leave it off the LRU).
*/
nr_reclaimed++;
continue;
}
}
}
if (PageAnon(page) && !PageSwapBacked(page)) {
/* follow __remove_mapping for reference */
if (!page_ref_freeze(page, 1))
goto keep_locked;
/*
* The page has only one reference left, which is
* from the isolation. After the caller puts the
* page back on lru and drops the reference, the
* page will be freed anyway. It doesn't matter
* which lru it goes. So we don't bother checking
* PageDirty here.
*/
count_vm_event(PGLAZYFREED);
count_memcg_page_event(page, PGLAZYFREED);
} else if (!mapping || !__remove_mapping(mapping, page, true,
sc->target_mem_cgroup))
goto keep_locked;
unlock_page(page);
free_it:
/*
* THP may get swapped out in a whole, need account
* all base pages.
*/
nr_reclaimed += nr_pages;
/*
* Is there need to periodically free_page_list? It would
* appear not as the counts should be low
*/
if (unlikely(PageTransHuge(page)))
destroy_compound_page(page);
else
list_add(&page->lru, &free_pages);
continue;
activate_locked_split:
/*
* The tail pages that are failed to add into swap cache
* reach here. Fixup nr_scanned and nr_pages.
*/
if (nr_pages > 1) {
sc->nr_scanned -= (nr_pages - 1);
nr_pages = 1;
}
activate_locked:
/* Not a candidate for swapping, so reclaim swap space. */
if (PageSwapCache(page) && (mem_cgroup_swap_full(page) ||
PageMlocked(page)))
try_to_free_swap(page);
VM_BUG_ON_PAGE(PageActive(page), page);
if (!PageMlocked(page)) {
int type = page_is_file_lru(page);
SetPageActive(page);
stat->nr_activate[type] += nr_pages;
count_memcg_page_event(page, PGACTIVATE);
}
keep_locked:
unlock_page(page);
keep:
list_add(&page->lru, &ret_pages);
VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
}
/* 'page_list' is always empty here */
/* Migrate pages selected for demotion */
nr_reclaimed += demote_page_list(&demote_pages, pgdat);
/* Pages that could not be demoted are still in @demote_pages */
if (!list_empty(&demote_pages)) {
/* Pages which failed to demoted go back on @page_list for retry: */
list_splice_init(&demote_pages, page_list);
do_demote_pass = false;
goto retry;
}
pgactivate = stat->nr_activate[0] + stat->nr_activate[1];
mem_cgroup_uncharge_list(&free_pages);
try_to_unmap_flush();
free_unref_page_list(&free_pages);
list_splice(&ret_pages, page_list);
count_vm_events(PGACTIVATE, pgactivate);
return nr_reclaimed;
}
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