[PATCH v4 26/73] page cache: Convert page cache lookups to XArray

From: Matthew Wilcox
Date: Tue Dec 05 2017 - 20:06:39 EST

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From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>

Introduce page_cache_pin() to factor out the common logic between the
various lookup routines:

find_get_entry
find_get_entries
find_get_pages_range
find_get_pages_contig
find_get_pages_range_tag
find_get_entries_tag
filemap_map_pages

By using the xa_state to control the iteration, we can remove most of
the gotos and just use the normal break/continue loop control flow.

Also convert the regression1 read-side to XArray since that simulates
the functions being modified here.

Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
---
include/linux/pagemap.h | 6 +-
mm/filemap.c | 380 +++++++++------------------------
tools/testing/radix-tree/regression1.c | 68 +++---
3 files changed, 129 insertions(+), 325 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 34d4fa3ad1c5..1a59f4a5424a 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -365,17 +365,17 @@ static inline unsigned find_get_pages(struct address_space *mapping,
unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t start,
unsigned int nr_pages, struct page **pages);
unsigned find_get_pages_range_tag(struct address_space *mapping, pgoff_t *index,
- pgoff_t end, int tag, unsigned int nr_pages,
+ pgoff_t end, xa_tag_t tag, unsigned int nr_pages,
struct page **pages);
static inline unsigned find_get_pages_tag(struct address_space *mapping,
- pgoff_t *index, int tag, unsigned int nr_pages,
+ pgoff_t *index, xa_tag_t tag, unsigned int nr_pages,
struct page **pages)
{
return find_get_pages_range_tag(mapping, index, (pgoff_t)-1, tag,
nr_pages, pages);
}
unsigned find_get_entries_tag(struct address_space *mapping, pgoff_t start,
- int tag, unsigned int nr_entries,
+ xa_tag_t tag, unsigned int nr_entries,
struct page **entries, pgoff_t *indices);

struct page *grab_cache_page_write_begin(struct address_space *mapping,
diff --git a/mm/filemap.c b/mm/filemap.c
index 6e2808fd3c06..6c9cad248e7f 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1401,6 +1401,32 @@ bool page_cache_range_empty(struct address_space *mapping, pgoff_t index,
}
EXPORT_SYMBOL_GPL(page_cache_range_empty);

+/*
+ * page_cache_pin() - Try to pin a page in the page cache.
+ * @xas: The XArray operation state.
+ * @pagep: The page which has been previously found at this location.
+ *
+ * On success, the page has an elevated refcount, but is not locked.
+ * This implements the lockless pagecache protocol as described in
+ * include/linux/pagemap.h; see page_cache_get_speculative().
+ *
+ * Return: True if the page is still in the cache.
+ */
+static bool page_cache_pin(struct xa_state *xas, struct page *page)
+{
+ struct page *head = compound_head(page);
+ bool got = page_cache_get_speculative(head);
+
+ if (likely(got && (xas_reload(xas) == page) &&
+ (compound_head(page) == head)))
+ return true;
+
+ if (got)
+ put_page(head);
+ xas_retry(xas, XA_RETRY_ENTRY);
+ return false;
+}
+
/**
* find_get_entry - find and get a page cache entry
* @mapping: the address_space to search
@@ -1416,51 +1442,21 @@ EXPORT_SYMBOL_GPL(page_cache_range_empty);
*/
struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)
{
- void **pagep;
- struct page *head, *page;
+ XA_STATE(xas, &mapping->pages, offset);
+ struct page *page;

rcu_read_lock();
-repeat:
- page = NULL;
- pagep = radix_tree_lookup_slot(&mapping->pages, offset);
- if (pagep) {
- page = radix_tree_deref_slot(pagep);
- if (unlikely(!page))
- goto out;
- if (radix_tree_exception(page)) {
- if (radix_tree_deref_retry(page))
- goto repeat;
- /*
- * A shadow entry of a recently evicted page,
- * or a swap entry from shmem/tmpfs. Return
- * it without attempting to raise page count.
- */
- 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);
- goto repeat;
- }
+ do {
+ page = xas_load(&xas);
+ if (xas_retry(&xas, page))
+ continue;
+ if (!page || xa_is_value(page))
+ break;;
+ if (!page_cache_pin(&xas, page))
+ continue;
+ } while (0);

- /*
- * Has the page moved?
- * This is part of the lockless pagecache protocol. See
- * include/linux/pagemap.h for details.
- */
- if (unlikely(page != *pagep)) {
- put_page(head);
- goto repeat;
- }
- }
-out:
rcu_read_unlock();
-
return page;
}
EXPORT_SYMBOL(find_get_entry);
@@ -1487,7 +1483,7 @@ struct page *find_lock_entry(struct address_space *mapping, pgoff_t offset)

repeat:
page = find_get_entry(mapping, offset);
- if (page && !radix_tree_exception(page)) {
+ if (page && !xa_is_value(page)) {
lock_page(page);
/* Has the page been truncated? */
if (unlikely(page_mapping(page) != mapping)) {
@@ -1620,50 +1616,21 @@ unsigned find_get_entries(struct address_space *mapping,
pgoff_t start, unsigned int nr_entries,
struct page **entries, pgoff_t *indices)
{
- void **slot;
+ XA_STATE(xas, &mapping->pages, start);
+ struct page *page;
unsigned int ret = 0;
- struct radix_tree_iter iter;

if (!nr_entries)
return 0;

rcu_read_lock();
- radix_tree_for_each_slot(slot, &mapping->pages, &iter, start) {
- struct page *head, *page;
-repeat:
- page = radix_tree_deref_slot(slot);
- if (unlikely(!page))
+ xas_for_each(&xas, page, ULONG_MAX) {
+ if (xas_retry(&xas, page))
+ continue;
+ if (!xa_is_value(page) && !page_cache_pin(&xas, page))
continue;
- if (radix_tree_exception(page)) {
- if (radix_tree_deref_retry(page)) {
- slot = radix_tree_iter_retry(&iter);
- continue;
- }
- /*
- * A shadow entry of a recently evicted page, a swap
- * entry from shmem/tmpfs or a DAX entry. Return it
- * without attempting to raise page count.
- */
- goto export;
- }
-
- 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);
- goto repeat;
- }

- /* Has the page moved? */
- if (unlikely(page != *slot)) {
- put_page(head);
- goto repeat;
- }
-export:
- indices[ret] = iter.index;
+ indices[ret] = xas.xa_index;
entries[ret] = page;
if (++ret == nr_entries)
break;
@@ -1697,56 +1664,26 @@ unsigned find_get_pages_range(struct address_space *mapping, pgoff_t *start,
pgoff_t end, unsigned int nr_pages,
struct page **pages)
{
- struct radix_tree_iter iter;
- void **slot;
+ XA_STATE(xas, &mapping->pages, *start);
+ struct page *page;
unsigned ret = 0;

if (unlikely(!nr_pages))
return 0;

rcu_read_lock();
- radix_tree_for_each_slot(slot, &mapping->pages, &iter, *start) {
- struct page *head, *page;
-
- if (iter.index > end)
- break;
-repeat:
- page = radix_tree_deref_slot(slot);
- if (unlikely(!page))
+ xas_for_each(&xas, page, end) {
+ if (xas_retry(&xas, page))
continue;
-
- if (radix_tree_exception(page)) {
- if (radix_tree_deref_retry(page)) {
- slot = radix_tree_iter_retry(&iter);
- continue;
- }
- /*
- * A shadow entry of a recently evicted page,
- * or a swap entry from shmem/tmpfs. Skip
- * over it.
- */
+ /* Skip over shadow or swap entries */
+ if (xa_is_value(page))
+ continue;
+ if (!page_cache_pin(&xas, page))
continue;
- }
-
- 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);
- goto repeat;
- }
-
- /* Has the page moved? */
- if (unlikely(page != *slot)) {
- put_page(head);
- goto repeat;
- }

pages[ret] = page;
if (++ret == nr_pages) {
- *start = pages[ret - 1]->index + 1;
+ *start = page->index + 1;
goto out;
}
}
@@ -1754,7 +1691,7 @@ unsigned find_get_pages_range(struct address_space *mapping, pgoff_t *start,
/*
* We come here when there is no page beyond @end. We take care to not
* overflow the index @start as it confuses some of the callers. This
- * breaks the iteration when there is page at index -1 but that is
+ * breaks the iteration when there is a page at index -1 but that is
* already broken anyway.
*/
if (end == (pgoff_t)-1)
@@ -1782,57 +1719,28 @@ unsigned find_get_pages_range(struct address_space *mapping, pgoff_t *start,
unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
unsigned int nr_pages, struct page **pages)
{
- struct radix_tree_iter iter;
- void **slot;
+ XA_STATE(xas, &mapping->pages, index);
+ struct page *page;
unsigned int ret = 0;

if (unlikely(!nr_pages))
return 0;

rcu_read_lock();
- radix_tree_for_each_contig(slot, &mapping->pages, &iter, index) {
- struct page *head, *page;
-repeat:
- page = radix_tree_deref_slot(slot);
- /* The hole, there no reason to continue */
- if (unlikely(!page))
- break;
-
- if (radix_tree_exception(page)) {
- if (radix_tree_deref_retry(page)) {
- slot = radix_tree_iter_retry(&iter);
- continue;
- }
- /*
- * A shadow entry of a recently evicted page,
- * or a swap entry from shmem/tmpfs. Stop
- * looking for contiguous pages.
- */
+ for (page = xas_load(&xas); page; page = xas_next(&xas)) {
+ if (xas_retry(&xas, page))
+ continue;
+ if (xa_is_value(page))
break;
- }
-
- 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);
- goto repeat;
- }
-
- /* Has the page moved? */
- if (unlikely(page != *slot)) {
- put_page(head);
- goto repeat;
- }
+ if (!page_cache_pin(&xas, page))
+ continue;

/*
* must check mapping and index after taking the ref.
* otherwise we can get both false positives and false
* negatives, which is just confusing to the caller.
*/
- if (page->mapping == NULL || page_to_pgoff(page) != iter.index) {
+ if (!page->mapping || page_to_pgoff(page) != xas.xa_index) {
put_page(page);
break;
}
@@ -1859,74 +1767,42 @@ EXPORT_SYMBOL(find_get_pages_contig);
* @tag. We update @index to index the next page for the traversal.
*/
unsigned find_get_pages_range_tag(struct address_space *mapping, pgoff_t *index,
- pgoff_t end, int tag, unsigned int nr_pages,
+ pgoff_t end, xa_tag_t tag, unsigned int nr_pages,
struct page **pages)
{
- struct radix_tree_iter iter;
- void **slot;
+ XA_STATE(xas, &mapping->pages, *index);
+ struct page *page;
unsigned ret = 0;

if (unlikely(!nr_pages))
return 0;

rcu_read_lock();
- radix_tree_for_each_tagged(slot, &mapping->pages, &iter, *index, tag) {
- struct page *head, *page;
-
- if (iter.index > end)
- break;
-repeat:
- page = radix_tree_deref_slot(slot);
- if (unlikely(!page))
+ xas_for_each_tag(&xas, page, end, tag) {
+ if (xas_retry(&xas, page))
continue;
-
- if (radix_tree_exception(page)) {
- if (radix_tree_deref_retry(page)) {
- slot = radix_tree_iter_retry(&iter);
- continue;
- }
- /*
- * A shadow entry of a recently evicted page.
- *
- * Those entries should never be tagged, but
- * this tree walk is lockless and the tags are
- * looked up in bulk, one radix tree node at a
- * time, so there is a sizable window for page
- * reclaim to evict a page we saw tagged.
- *
- * Skip over it.
- */
+ /*
+ * Shadow entries should never be tagged, but this tree walk
+ * is lockless so there is a window for page reclaim to evict
+ * a page we saw tagged. Skip over it.
+ */
+ if (xa_is_value(page))
+ continue;
+ if (!page_cache_pin(&xas, page))
continue;
- }
-
- 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);
- goto repeat;
- }
-
- /* Has the page moved? */
- if (unlikely(page != *slot)) {
- put_page(head);
- goto repeat;
- }

pages[ret] = page;
if (++ret == nr_pages) {
- *index = pages[ret - 1]->index + 1;
+ *index = page->index + 1;
goto out;
}
}

/*
- * We come here when we got at @end. We take care to not overflow the
+ * We come here when we got to @end. We take care to not overflow the
* index @index as it confuses some of the callers. This breaks the
- * iteration when there is page at index -1 but that is already broken
- * anyway.
+ * iteration when there is a page at index -1 but that is already
+ * broken anyway.
*/
if (end == (pgoff_t)-1)
*index = (pgoff_t)-1;
@@ -1952,54 +1828,24 @@ EXPORT_SYMBOL(find_get_pages_range_tag);
* @tag.
*/
unsigned find_get_entries_tag(struct address_space *mapping, pgoff_t start,
- int tag, unsigned int nr_entries,
+ xa_tag_t tag, unsigned int nr_entries,
struct page **entries, pgoff_t *indices)
{
- void **slot;
+ XA_STATE(xas, &mapping->pages, start);
+ struct page *page;
unsigned int ret = 0;
- struct radix_tree_iter iter;

if (!nr_entries)
return 0;

rcu_read_lock();
- radix_tree_for_each_tagged(slot, &mapping->pages, &iter, start, tag) {
- struct page *head, *page;
-repeat:
- page = radix_tree_deref_slot(slot);
- if (unlikely(!page))
+ xas_for_each_tag(&xas, page, ULONG_MAX, tag) {
+ if (xas_retry(&xas, page))
+ continue;
+ if (!xa_is_value(page) && !page_cache_pin(&xas, page))
continue;
- if (radix_tree_exception(page)) {
- if (radix_tree_deref_retry(page)) {
- slot = radix_tree_iter_retry(&iter);
- continue;
- }
-
- /*
- * A shadow entry of a recently evicted page, a swap
- * entry from shmem/tmpfs or a DAX entry. Return it
- * without attempting to raise page count.
- */
- goto export;
- }
-
- 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);
- goto repeat;
- }

- /* Has the page moved? */
- if (unlikely(page != *slot)) {
- put_page(head);
- goto repeat;
- }
-export:
- indices[ret] = iter.index;
+ indices[ret] = xas.xa_index;
entries[ret] = page;
if (++ret == nr_entries)
break;
@@ -2608,45 +2454,21 @@ EXPORT_SYMBOL(filemap_fault);
void filemap_map_pages(struct vm_fault *vmf,
pgoff_t start_pgoff, pgoff_t end_pgoff)
{
- struct radix_tree_iter iter;
- void **slot;
struct file *file = vmf->vma->vm_file;
struct address_space *mapping = file->f_mapping;
pgoff_t last_pgoff = start_pgoff;
unsigned long max_idx;
- struct page *head, *page;
+ XA_STATE(xas, &mapping->pages, start_pgoff);
+ struct page *page;

rcu_read_lock();
- radix_tree_for_each_slot(slot, &mapping->pages, &iter, start_pgoff) {
- if (iter.index > end_pgoff)
- break;
-repeat:
- page = radix_tree_deref_slot(slot);
- if (unlikely(!page))
- goto next;
- if (radix_tree_exception(page)) {
- if (radix_tree_deref_retry(page)) {
- slot = radix_tree_iter_retry(&iter);
- continue;
- }
+ xas_for_each(&xas, page, end_pgoff) {
+ if (xas_retry(&xas, page))
+ continue;
+ if (xa_is_value(page))
goto next;
- }
-
- 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);
- goto repeat;
- }
-
- /* Has the page moved? */
- if (unlikely(page != *slot)) {
- put_page(head);
- goto repeat;
- }
+ if (!page_cache_pin(&xas, page))
+ continue;

if (!PageUptodate(page) ||
PageReadahead(page) ||
@@ -2665,10 +2487,10 @@ void filemap_map_pages(struct vm_fault *vmf,
if (file->f_ra.mmap_miss > 0)
file->f_ra.mmap_miss--;

- vmf->address += (iter.index - last_pgoff) << PAGE_SHIFT;
+ vmf->address += (xas.xa_index - last_pgoff) << PAGE_SHIFT;
if (vmf->pte)
- vmf->pte += iter.index - last_pgoff;
- last_pgoff = iter.index;
+ vmf->pte += xas.xa_index - last_pgoff;
+ last_pgoff = xas.xa_index;
if (alloc_set_pte(vmf, NULL, page))
goto unlock;
unlock_page(page);
@@ -2681,8 +2503,6 @@ void filemap_map_pages(struct vm_fault *vmf,
/* Huge page is mapped? No need to proceed. */
if (pmd_trans_huge(*vmf->pmd))
break;
- if (iter.index == end_pgoff)
- break;
}
rcu_read_unlock();
}
diff --git a/tools/testing/radix-tree/regression1.c b/tools/testing/radix-tree/regression1.c
index 0aece092f40e..008393906be5 100644
--- a/tools/testing/radix-tree/regression1.c
+++ b/tools/testing/radix-tree/regression1.c
@@ -58,7 +58,7 @@ static struct page *page_alloc(void)
struct page *p;
p = malloc(sizeof(struct page));
p->count = 1;
- p->index = 1;
+ p->index = (unsigned long)p;
pthread_mutex_init(&p->lock, NULL);

return p;
@@ -77,53 +77,37 @@ static void page_free(struct page *p)
call_rcu(&p->rcu, page_rcu_free);
}

+static bool page_cache_pin(struct xa_state *xas, struct page *page)
+{
+ pthread_mutex_lock(&page->lock);
+ if (!page->count) {
+ pthread_mutex_unlock(&page->lock);
+ goto fail;
+ }
+ /* don't actually update page refcount */
+ pthread_mutex_unlock(&page->lock);
+
+ /* Has the page moved? */
+ if (xas_reload(xas) == page)
+ return true;
+fail:
+ xas_retry(xas, XA_RETRY_ENTRY);
+ return false;
+}
+
static unsigned find_get_pages(unsigned long start,
unsigned int nr_pages, struct page **pages)
{
- unsigned int i;
- unsigned int ret;
- unsigned int nr_found;
+ XA_STATE(xas, &mt_tree, start);
+ struct page *page;
+ unsigned int ret = 0;

rcu_read_lock();
-restart:
- nr_found = radix_tree_gang_lookup_slot(&mt_tree,
- (void ***)pages, NULL, start, nr_pages);
- ret = 0;
- for (i = 0; i < nr_found; i++) {
- struct page *page;
-repeat:
- page = radix_tree_deref_slot((void **)pages[i]);
- if (unlikely(!page))
+ xas_for_each(&xas, page, ULONG_MAX) {
+ if (xas_retry(&xas, page))
+ continue;
+ if (!page_cache_pin(&xas, page))
continue;
-
- if (radix_tree_exception(page)) {
- if (radix_tree_deref_retry(page)) {
- /*
- * Transient condition which can only trigger
- * when entry at index 0 moves out of or back
- * to root: none yet gotten, safe to restart.
- */
- assert((start | i) == 0);
- goto restart;
- }
- /*
- * No exceptional entries are inserted in this test.
- */
- assert(0);
- }
-
- pthread_mutex_lock(&page->lock);
- if (!page->count) {
- pthread_mutex_unlock(&page->lock);
- goto repeat;
- }
- /* don't actually update page refcount */
- pthread_mutex_unlock(&page->lock);
-
- /* Has the page moved? */
- if (unlikely(page != *((void **)pages[i]))) {
- goto repeat;
- }

pages[ret] = page;
ret++;
--
2.15.0

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