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Subject[PATCH v3] page cache: Store only head pages in i_pages
Date
Transparent Huge Pages are currently stored in i_pages as pointers to
consecutive subpages. This patch changes that to storing consecutive
pointers to the head page in preparation for storing huge pages more
efficiently in i_pages.

Large parts of this are "inspired" by Kirill's patch
https://lore.kernel.org/lkml/20170126115819.58875-2-kirill.shutemov@linux.intel.com/

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Acked-by: Jan Kara <jack@suse.cz>
Reviewed-by: Kirill Shutemov <kirill@shutemov.name>
---
include/linux/pagemap.h | 9 +++
mm/filemap.c | 158 ++++++++++++++++------------------------
mm/huge_memory.c | 3 +
mm/khugepaged.c | 4 +-
mm/memfd.c | 2 +
mm/migrate.c | 2 +-
mm/shmem.c | 2 +-
mm/swap_state.c | 4 +-
8 files changed, 81 insertions(+), 103 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index bcf909d0de5f..7d58e4e0b68e 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -333,6 +333,15 @@ static inline struct page *grab_cache_page_nowait(struct address_space *mapping,
mapping_gfp_mask(mapping));
}

+static inline struct page *find_subpage(struct page *page, pgoff_t offset)
+{
+ VM_BUG_ON_PAGE(PageTail(page), page);
+ VM_BUG_ON_PAGE(page->index > offset, page);
+ VM_BUG_ON_PAGE(page->index + (1 << compound_order(page)) <= offset,
+ page);
+ return page - page->index + offset;
+}
+
struct page *find_get_entry(struct address_space *mapping, pgoff_t offset);
struct page *find_lock_entry(struct address_space *mapping, pgoff_t offset);
unsigned find_get_entries(struct address_space *mapping, pgoff_t start,
diff --git a/mm/filemap.c b/mm/filemap.c
index 5673672fd444..d9161cae11b5 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -279,11 +279,11 @@ EXPORT_SYMBOL(delete_from_page_cache);
* @pvec: pagevec with pages to delete
*
* The function walks over mapping->i_pages and removes pages passed in @pvec
- * from the mapping. The function expects @pvec to be sorted by page index.
+ * from the mapping. The function expects @pvec to be sorted by page index
+ * and is optimised for it to be dense.
* It tolerates holes in @pvec (mapping entries at those indices are not
* modified). The function expects only THP head pages to be present in the
- * @pvec and takes care to delete all corresponding tail pages from the
- * mapping as well.
+ * @pvec.
*
* The function expects the i_pages lock to be held.
*/
@@ -292,40 +292,43 @@ static void page_cache_delete_batch(struct address_space *mapping,
{
XA_STATE(xas, &mapping->i_pages, pvec->pages[0]->index);
int total_pages = 0;
- int i = 0, tail_pages = 0;
+ int i = 0;
struct page *page;

mapping_set_update(&xas, mapping);
xas_for_each(&xas, page, ULONG_MAX) {
- if (i >= pagevec_count(pvec) && !tail_pages)
+ if (i >= pagevec_count(pvec))
break;
+
+ /* A swap/dax/shadow entry got inserted? Skip it. */
if (xa_is_value(page))
continue;
- if (!tail_pages) {
- /*
- * Some page got inserted in our range? Skip it. We
- * have our pages locked so they are protected from
- * being removed.
- */
- if (page != pvec->pages[i]) {
- VM_BUG_ON_PAGE(page->index >
- pvec->pages[i]->index, page);
- continue;
- }
- WARN_ON_ONCE(!PageLocked(page));
- if (PageTransHuge(page) && !PageHuge(page))
- tail_pages = HPAGE_PMD_NR - 1;
+ /*
+ * A page got inserted in our range? Skip it. We have our
+ * pages locked so they are protected from being removed.
+ * If we see a page whose index is higher than ours, it
+ * means our page has been removed, which shouldn't be
+ * possible because we're holding the PageLock.
+ */
+ if (page != pvec->pages[i]) {
+ VM_BUG_ON_PAGE(page->index > pvec->pages[i]->index,
+ page);
+ continue;
+ }
+
+ WARN_ON_ONCE(!PageLocked(page));
+
+ if (page->index == xas.xa_index)
page->mapping = NULL;
- /*
- * Leave page->index set: truncation lookup relies
- * upon it
- */
+ /* Leave page->index set: truncation lookup relies on it */
+
+ /*
+ * Move to the next page in the vector if this is a small page
+ * or the index is of the last page in this compound page).
+ */
+ if (page->index + (1UL << compound_order(page)) - 1 ==
+ xas.xa_index)
i++;
- } else {
- VM_BUG_ON_PAGE(page->index + HPAGE_PMD_NR - tail_pages
- != pvec->pages[i]->index, page);
- tail_pages--;
- }
xas_store(&xas, NULL);
total_pages++;
}
@@ -1491,7 +1494,7 @@ EXPORT_SYMBOL(page_cache_prev_miss);
struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)
{
XA_STATE(xas, &mapping->i_pages, offset);
- struct page *head, *page;
+ struct page *page;

rcu_read_lock();
repeat:
@@ -1506,25 +1509,19 @@ struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)
if (!page || xa_is_value(page))
goto out;

- head = compound_head(page);
- if (!page_cache_get_speculative(head))
+ if (!page_cache_get_speculative(page))
goto repeat;

- /* The page was split under us? */
- if (compound_head(page) != head) {
- put_page(head);
- goto repeat;
- }
-
/*
- * Has the page moved?
+ * Has the page moved or been split?
* This is part of the lockless pagecache protocol. See
* include/linux/pagemap.h for details.
*/
if (unlikely(page != xas_reload(&xas))) {
- put_page(head);
+ put_page(page);
goto repeat;
}
+ page = find_subpage(page, offset);
out:
rcu_read_unlock();

@@ -1706,7 +1703,6 @@ unsigned find_get_entries(struct address_space *mapping,

rcu_read_lock();
xas_for_each(&xas, page, ULONG_MAX) {
- struct page *head;
if (xas_retry(&xas, page))
continue;
/*
@@ -1717,17 +1713,13 @@ unsigned find_get_entries(struct address_space *mapping,
if (xa_is_value(page))
goto export;

- head = compound_head(page);
- if (!page_cache_get_speculative(head))
+ if (!page_cache_get_speculative(page))
goto retry;

- /* The page was split under us? */
- if (compound_head(page) != head)
- goto put_page;
-
- /* Has the page moved? */
+ /* Has the page moved or been split? */
if (unlikely(page != xas_reload(&xas)))
goto put_page;
+ page = find_subpage(page, xas.xa_index);

export:
indices[ret] = xas.xa_index;
@@ -1736,7 +1728,7 @@ unsigned find_get_entries(struct address_space *mapping,
break;
continue;
put_page:
- put_page(head);
+ put_page(page);
retry:
xas_reset(&xas);
}
@@ -1778,33 +1770,27 @@ unsigned find_get_pages_range(struct address_space *mapping, pgoff_t *start,

rcu_read_lock();
xas_for_each(&xas, page, end) {
- struct page *head;
if (xas_retry(&xas, page))
continue;
/* Skip over shadow, swap and DAX entries */
if (xa_is_value(page))
continue;

- head = compound_head(page);
- if (!page_cache_get_speculative(head))
+ if (!page_cache_get_speculative(page))
goto retry;

- /* The page was split under us? */
- if (compound_head(page) != head)
- goto put_page;
-
- /* Has the page moved? */
+ /* Has the page moved or been split? */
if (unlikely(page != xas_reload(&xas)))
goto put_page;

- pages[ret] = page;
+ pages[ret] = find_subpage(page, xas.xa_index);
if (++ret == nr_pages) {
*start = page->index + 1;
goto out;
}
continue;
put_page:
- put_page(head);
+ put_page(page);
retry:
xas_reset(&xas);
}
@@ -1849,7 +1835,6 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,

rcu_read_lock();
for (page = xas_load(&xas); page; page = xas_next(&xas)) {
- struct page *head;
if (xas_retry(&xas, page))
continue;
/*
@@ -1859,24 +1844,19 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
if (xa_is_value(page))
break;

- head = compound_head(page);
- if (!page_cache_get_speculative(head))
+ if (!page_cache_get_speculative(page))
goto retry;

- /* The page was split under us? */
- if (compound_head(page) != head)
- goto put_page;
-
- /* Has the page moved? */
+ /* Has the page moved or been split? */
if (unlikely(page != xas_reload(&xas)))
goto put_page;

- pages[ret] = page;
+ pages[ret] = find_subpage(page, xas.xa_index);
if (++ret == nr_pages)
break;
continue;
put_page:
- put_page(head);
+ put_page(page);
retry:
xas_reset(&xas);
}
@@ -1912,7 +1892,6 @@ unsigned find_get_pages_range_tag(struct address_space *mapping, pgoff_t *index,

rcu_read_lock();
xas_for_each_marked(&xas, page, end, tag) {
- struct page *head;
if (xas_retry(&xas, page))
continue;
/*
@@ -1923,26 +1902,21 @@ unsigned find_get_pages_range_tag(struct address_space *mapping, pgoff_t *index,
if (xa_is_value(page))
continue;

- head = compound_head(page);
- if (!page_cache_get_speculative(head))
+ if (!page_cache_get_speculative(page))
goto retry;

- /* The page was split under us? */
- if (compound_head(page) != head)
- goto put_page;
-
- /* Has the page moved? */
+ /* Has the page moved or been split? */
if (unlikely(page != xas_reload(&xas)))
goto put_page;

- pages[ret] = page;
+ pages[ret] = find_subpage(page, xas.xa_index);
if (++ret == nr_pages) {
*index = page->index + 1;
goto out;
}
continue;
put_page:
- put_page(head);
+ put_page(page);
retry:
xas_reset(&xas);
}
@@ -1991,7 +1965,6 @@ unsigned find_get_entries_tag(struct address_space *mapping, pgoff_t start,

rcu_read_lock();
xas_for_each_marked(&xas, page, ULONG_MAX, tag) {
- struct page *head;
if (xas_retry(&xas, page))
continue;
/*
@@ -2002,17 +1975,13 @@ unsigned find_get_entries_tag(struct address_space *mapping, pgoff_t start,
if (xa_is_value(page))
goto export;

- head = compound_head(page);
- if (!page_cache_get_speculative(head))
+ if (!page_cache_get_speculative(page))
goto retry;

- /* The page was split under us? */
- if (compound_head(page) != head)
- goto put_page;
-
- /* Has the page moved? */
+ /* Has the page moved or been split? */
if (unlikely(page != xas_reload(&xas)))
goto put_page;
+ page = find_subpage(page, xas.xa_index);

export:
indices[ret] = xas.xa_index;
@@ -2021,7 +1990,7 @@ unsigned find_get_entries_tag(struct address_space *mapping, pgoff_t start,
break;
continue;
put_page:
- put_page(head);
+ put_page(page);
retry:
xas_reset(&xas);
}
@@ -2686,7 +2655,7 @@ void filemap_map_pages(struct vm_fault *vmf,
pgoff_t last_pgoff = start_pgoff;
unsigned long max_idx;
XA_STATE(xas, &mapping->i_pages, start_pgoff);
- struct page *head, *page;
+ struct page *page;

rcu_read_lock();
xas_for_each(&xas, page, end_pgoff) {
@@ -2695,24 +2664,19 @@ void filemap_map_pages(struct vm_fault *vmf,
if (xa_is_value(page))
goto next;

- head = compound_head(page);
-
/*
* Check for a locked page first, as a speculative
* reference may adversely influence page migration.
*/
- if (PageLocked(head))
+ if (PageLocked(page))
goto next;
- if (!page_cache_get_speculative(head))
+ if (!page_cache_get_speculative(page))
goto next;

- /* The page was split under us? */
- if (compound_head(page) != head)
- goto skip;
-
- /* Has the page moved? */
+ /* Has the page moved or been split? */
if (unlikely(page != xas_reload(&xas)))
goto skip;
+ page = find_subpage(page, xas.xa_index);

if (!PageUptodate(page) ||
PageReadahead(page) ||
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d4847026d4b1..7008174c033b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2458,6 +2458,9 @@ static void __split_huge_page(struct page *page, struct list_head *list,
if (IS_ENABLED(CONFIG_SHMEM) && PageSwapBacked(head))
shmem_uncharge(head->mapping->host, 1);
put_page(head + i);
+ } else if (!PageAnon(page)) {
+ __xa_store(&head->mapping->i_pages, head[i].index,
+ head + i, 0);
}
}

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 449044378782..7ba7a1e4fa79 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1374,7 +1374,7 @@ static void collapse_shmem(struct mm_struct *mm,
result = SCAN_FAIL;
goto xa_locked;
}
- xas_store(&xas, new_page + (index % HPAGE_PMD_NR));
+ xas_store(&xas, new_page);
nr_none++;
continue;
}
@@ -1450,7 +1450,7 @@ static void collapse_shmem(struct mm_struct *mm,
list_add_tail(&page->lru, &pagelist);

/* Finally, replace with the new page. */
- xas_store(&xas, new_page + (index % HPAGE_PMD_NR));
+ xas_store(&xas, new_page);
continue;
out_unlock:
unlock_page(page);
diff --git a/mm/memfd.c b/mm/memfd.c
index 650e65a46b9c..bccbf7dff050 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -39,6 +39,7 @@ static void memfd_tag_pins(struct xa_state *xas)
xas_for_each(xas, page, ULONG_MAX) {
if (xa_is_value(page))
continue;
+ page = find_subpage(page, xas.xa_index);
if (page_count(page) - page_mapcount(page) > 1)
xas_set_mark(xas, MEMFD_TAG_PINNED);

@@ -88,6 +89,7 @@ static int memfd_wait_for_pins(struct address_space *mapping)
bool clear = true;
if (xa_is_value(page))
continue;
+ page = find_subpage(page, xas.xa_index);
if (page_count(page) - page_mapcount(page) != 1) {
/*
* On the last scan, we clean up all those tags
diff --git a/mm/migrate.c b/mm/migrate.c
index 412d5fff78d4..8cb55dd69b9c 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -465,7 +465,7 @@ int migrate_page_move_mapping(struct address_space *mapping,

for (i = 1; i < HPAGE_PMD_NR; i++) {
xas_next(&xas);
- xas_store(&xas, newpage + i);
+ xas_store(&xas, newpage);
}
}

diff --git a/mm/shmem.c b/mm/shmem.c
index c8cdaa012f18..a78d4f05a51f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -614,7 +614,7 @@ static int shmem_add_to_page_cache(struct page *page,
if (xas_error(&xas))
goto unlock;
next:
- xas_store(&xas, page + i);
+ xas_store(&xas, page);
if (++i < nr) {
xas_next(&xas);
goto next;
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 85245fdec8d9..eb714165afd2 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -132,7 +132,7 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp)
for (i = 0; i < nr; i++) {
VM_BUG_ON_PAGE(xas.xa_index != idx + i, page);
set_page_private(page + i, entry.val + i);
- xas_store(&xas, page + i);
+ xas_store(&xas, page);
xas_next(&xas);
}
address_space->nrpages += nr;
@@ -167,7 +167,7 @@ void __delete_from_swap_cache(struct page *page, swp_entry_t entry)

for (i = 0; i < nr; i++) {
void *entry = xas_store(&xas, NULL);
- VM_BUG_ON_PAGE(entry != page + i, entry);
+ VM_BUG_ON_PAGE(entry != page, entry);
set_page_private(page + i, 0);
xas_next(&xas);
}
--
2.20.1
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