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Subject[PATCH 2/2] 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>
---
include/linux/pagemap.h | 9 ++++
mm/filemap.c | 96 +++++++++++++----------------------------
mm/khugepaged.c | 4 +-
mm/shmem.c | 2 +-
mm/swap_state.c | 2 +-
5 files changed, 42 insertions(+), 71 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 226f96f0dee06..41bf976574e74 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -345,6 +345,15 @@ 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 538531590ef2d..d7274591381ac 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1407,7 +1407,7 @@ 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:
@@ -1422,25 +1422,19 @@ 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))
- goto repeat;
-
- /* The page was split under us? */
- if (compound_head(page) != head) {
- put_page(head);
+ if (!page_cache_get_speculative(page))
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();

@@ -1611,7 +1605,6 @@ 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;
/*
@@ -1622,17 +1615,13 @@ 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;
@@ -1641,7 +1630,7 @@ unsigned find_get_entries(struct address_space *mapping,
break;
continue;
put_page:
- put_page(head);
+ put_page(page);
retry:
xas_reset(&xas);
}
@@ -1683,33 +1672,27 @@ 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);
}
@@ -1754,7 +1737,6 @@ 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;
/*
@@ -1764,24 +1746,19 @@ 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);
}
@@ -1815,7 +1792,6 @@ 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;
/*
@@ -1826,26 +1802,21 @@ 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);
}
@@ -1892,7 +1863,6 @@ 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;
/*
@@ -1903,17 +1873,13 @@ 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;
@@ -1922,7 +1888,7 @@ 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);
}
@@ -2533,7 +2499,7 @@ 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) {
@@ -2542,17 +2508,13 @@ void filemap_map_pages(struct vm_fault *vmf,
if (xa_is_value(page))
goto next;

- head = compound_head(page);
- 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/khugepaged.c b/mm/khugepaged.c
index c13625c1ad5e5..7d6a1319dd42e 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1363,7 +1363,7 @@ static void collapse_shmem(struct mm_struct *mm,
result = SCAN_FAIL;
break;
}
- xas_store(&xas, new_page + (index % HPAGE_PMD_NR));
+ xas_store(&xas, new_page);
nr_none++;
continue;
}
@@ -1431,7 +1431,7 @@ 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_lru:
xas_unlock_irq(&xas);
diff --git a/mm/shmem.c b/mm/shmem.c
index ea26d7a0342d7..c2ba84cbb0c0e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -603,7 +603,7 @@ 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 fd2f21e1c60ae..dcf9e466d2945 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;
--
2.19.1
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