[RFC][PATCH] mm: Split page_has_private() in two to better handle PG_private_2

From: David Howells
Date: Thu Apr 08 2021 - 17:15:44 EST

Hi Willy, Linus,

How about this to handle the situation with PG_private_2? I think it handles
things according to Linus's suggestion.

David
---
mm: Split page_has_private() in two to better handle PG_private_2

Split page_has_private() into two functions:

(1) page_needs_cleanup() to find out if a page needs the ->releasepage(),
->invalidatepage(), etc. address space ops calling upon it.

This returns true when either PG_private or PG_private_2 are set.

(2) page_private_count() which returns a count of the number of refs
contributed to a page for attached private data.

This returns 1 if PG_private is set and 0 otherwise.

I think the suggestion[1] is that PG_private_2 should just have a ref on
the page, but this isn't accounted in the same way as PG_private's ref.

Notes:

(*) The following:

btrfs_migratepage()
iomap_set_range_uptodate()
iomap_migrate_page()
to_iomap_page()

should probably all use PagePrivate() rather than page_has_private()
since they're interested in what's attached to page->private when
they're doing this, and not PG_private_2.

It may not matter in these cases since page->private is probably NULL
if PG_private is not set.

(*) Do we actually need PG_private, or is it possible just to see if
page->private is NULL?

(*) There's a lot of "if (page_has_private()) try_to_release_page()"
combos. Does it make sense to create a inline function for this?

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Link: https://lore.kernel.org/linux-fsdevel/CAHk-=whWoJhGeMn85LOh9FX-5d2-Upzmv1m2ZmYxvD31TKpUTA@xxxxxxxxxxxxxx/ [1]
---
fs/btrfs/disk-io.c | 2 +-
fs/btrfs/inode.c | 2 +-
fs/ext4/move_extent.c | 8 ++++----
fs/fuse/dev.c | 2 +-
fs/iomap/buffered-io.c | 6 +++---
fs/splice.c | 2 +-
include/linux/page-flags.h | 17 +++++++++++++++--
include/trace/events/pagemap.h | 2 +-
mm/khugepaged.c | 4 ++--
mm/migrate.c | 10 +++++-----
mm/readahead.c | 2 +-
mm/truncate.c | 12 ++++++------
mm/vmscan.c | 12 ++++++------
13 files changed, 47 insertions(+), 34 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 41b718cfea40..d95f8d4b3004 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -936,7 +936,7 @@ static int btree_migratepage(struct address_space *mapping,
* Buffers may be managed in a filesystem specific way.
* We must have no buffers or drop them.
*/
- if (page_has_private(page) &&
+ if (page_needs_cleanup(page) &&
!try_to_release_page(page, GFP_KERNEL))
return -EAGAIN;
return migrate_page(mapping, newpage, page, mode);
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 7cdf65be3707..94f038d34f16 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -8333,7 +8333,7 @@ static int btrfs_migratepage(struct address_space *mapping,
if (ret != MIGRATEPAGE_SUCCESS)
return ret;

- if (page_has_private(page))
+ if (PagePrivate(page))
attach_page_private(newpage, detach_page_private(page));

if (PagePrivate2(page)) {
diff --git a/fs/ext4/move_extent.c b/fs/ext4/move_extent.c
index 64a579734f93..16d0a7a73191 100644
--- a/fs/ext4/move_extent.c
+++ b/fs/ext4/move_extent.c
@@ -329,9 +329,9 @@ move_extent_per_page(struct file *o_filp, struct inode *donor_inode,
ext4_double_up_write_data_sem(orig_inode, donor_inode);
goto data_copy;
}
- if ((page_has_private(pagep[0]) &&
+ if ((page_needs_cleanup(pagep[0]) &&
!try_to_release_page(pagep[0], 0)) ||
- (page_has_private(pagep[1]) &&
+ (page_needs_cleanup(pagep[1]) &&
!try_to_release_page(pagep[1], 0))) {
*err = -EBUSY;
goto drop_data_sem;
@@ -351,8 +351,8 @@ move_extent_per_page(struct file *o_filp, struct inode *donor_inode,

/* At this point all buffers in range are uptodate, old mapping layout
* is no longer required, try to drop it now. */
- if ((page_has_private(pagep[0]) && !try_to_release_page(pagep[0], 0)) ||
- (page_has_private(pagep[1]) && !try_to_release_page(pagep[1], 0))) {
+ if ((page_needs_cleanup(pagep[0]) && !try_to_release_page(pagep[0], 0)) ||
+ (page_needs_cleanup(pagep[1]) && !try_to_release_page(pagep[1], 0))) {
*err = -EBUSY;
goto unlock_pages;
}
diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index c0fee830a34e..76e8ca9e47fa 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -837,7 +837,7 @@ static int fuse_try_move_page(struct fuse_copy_state *cs, struct page **pagep)
*/
if (WARN_ON(page_mapped(oldpage)))
goto out_fallback_unlock;
- if (WARN_ON(page_has_private(oldpage)))
+ if (WARN_ON(page_needs_cleanup(oldpage)))
goto out_fallback_unlock;
if (WARN_ON(PageDirty(oldpage) || PageWriteback(oldpage)))
goto out_fallback_unlock;
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 414769a6ad11..9c89db033548 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -41,7 +41,7 @@ static inline struct iomap_page *to_iomap_page(struct page *page)
*/
VM_BUG_ON_PGFLAGS(PageTail(page), page);

- if (page_has_private(page))
+ if (page_needs_cleanup(page))
return (struct iomap_page *)page_private(page);
return NULL;
}
@@ -163,7 +163,7 @@ iomap_set_range_uptodate(struct page *page, unsigned off, unsigned len)
if (PageError(page))
return;

- if (page_has_private(page))
+ if (PagePrivate(page))
iomap_iop_set_range_uptodate(page, off, len);
else
SetPageUptodate(page);
@@ -502,7 +502,7 @@ iomap_migrate_page(struct address_space *mapping, struct page *newpage,
if (ret != MIGRATEPAGE_SUCCESS)
return ret;

- if (page_has_private(page))
+ if (PagePrivate(page))
attach_page_private(newpage, detach_page_private(page));

if (mode != MIGRATE_SYNC_NO_COPY)
diff --git a/fs/splice.c b/fs/splice.c
index 5dbce4dcc1a7..bf102bc947bb 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -65,7 +65,7 @@ static bool page_cache_pipe_buf_try_steal(struct pipe_inode_info *pipe,
*/
wait_on_page_writeback(page);

- if (page_has_private(page) &&
+ if (page_needs_cleanup(page) &&
!try_to_release_page(page, GFP_KERNEL))
goto out_unlock;

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 04a34c08e0a6..04cb440ce06e 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -832,14 +832,27 @@ static inline void ClearPageSlabPfmemalloc(struct page *page)

#define PAGE_FLAGS_PRIVATE \
(1UL << PG_private | 1UL << PG_private_2)
+
+/**
+ * page_private_count - Find out how many refs a page's private data contribute
+ * @page: The page to be checked
+ *
+ * Return the contribution to the pagecount of the private data attached to a
+ * page.
+ */
+static inline int page_private_count(struct page *page)
+{
+ return test_bit(PG_private, &page->flags) ? 1 : 0;
+}
+
/**
- * page_has_private - Determine if page has private stuff
+ * page_needs_cleanup - Determine if page has private stuff that needs cleaning
* @page: The page to be checked
*
* Determine if a page has private stuff, indicating that release routines
* should be invoked upon it.
*/
-static inline int page_has_private(struct page *page)
+static inline int page_needs_cleanup(struct page *page)
{
return !!(page->flags & PAGE_FLAGS_PRIVATE);
}
diff --git a/include/trace/events/pagemap.h b/include/trace/events/pagemap.h
index e1735fe7c76a..3ff3404cc399 100644
--- a/include/trace/events/pagemap.h
+++ b/include/trace/events/pagemap.h
@@ -22,7 +22,7 @@
(PageSwapCache(page) ? PAGEMAP_SWAPCACHE : 0) | \
(PageSwapBacked(page) ? PAGEMAP_SWAPBACKED : 0) | \
(PageMappedToDisk(page) ? PAGEMAP_MAPPEDDISK : 0) | \
- (page_has_private(page) ? PAGEMAP_BUFFERS : 0) \
+ (page_needs_cleanup(page) ? PAGEMAP_BUFFERS : 0) \
)

TRACE_EVENT(mm_lru_insertion,
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index a7d6cb912b05..d3b60a31aae2 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1822,7 +1822,7 @@ static void collapse_file(struct mm_struct *mm,
goto out_unlock;
}

- if (page_has_private(page) &&
+ if (page_needs_cleanup(page) &&
!try_to_release_page(page, GFP_KERNEL)) {
result = SCAN_PAGE_HAS_PRIVATE;
putback_lru_page(page);
@@ -2019,7 +2019,7 @@ static void khugepaged_scan_file(struct mm_struct *mm,
}

if (page_count(page) !=
- 1 + page_mapcount(page) + page_has_private(page)) {
+ 1 + page_mapcount(page) + page_private_count(page)) {
result = SCAN_PAGE_COUNT;
break;
}
diff --git a/mm/migrate.c b/mm/migrate.c
index 62b81d5257aa..eafd73bea945 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -382,7 +382,7 @@ static int expected_page_refs(struct address_space *mapping, struct page *page)
*/
expected_count += is_device_private_page(page);
if (mapping)
- expected_count += thp_nr_pages(page) + page_has_private(page);
+ expected_count += thp_nr_pages(page) + page_private_count(page);

return expected_count;
}
@@ -530,7 +530,7 @@ int migrate_huge_page_move_mapping(struct address_space *mapping,
int expected_count;

xas_lock_irq(&xas);
- expected_count = 2 + page_has_private(page);
+ expected_count = 2 + page_private_count(page);
if (page_count(page) != expected_count || xas_load(&xas) != page) {
xas_unlock_irq(&xas);
return -EAGAIN;
@@ -924,7 +924,7 @@ static int fallback_migrate_page(struct address_space *mapping,
* Buffers may be managed in a filesystem specific way.
* We must have no buffers or drop them.
*/
- if (page_has_private(page) &&
+ if (page_needs_cleanup(page) &&
!try_to_release_page(page, GFP_KERNEL))
return mode == MIGRATE_SYNC ? -EAGAIN : -EBUSY;

@@ -1117,7 +1117,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
*/
if (!page->mapping) {
VM_BUG_ON_PAGE(PageAnon(page), page);
- if (page_has_private(page)) {
+ if (page_needs_cleanup(page)) {
try_to_free_buffers(page);
goto out_unlock_both;
}
@@ -2618,7 +2618,7 @@ static bool migrate_vma_check_page(struct page *page)

/* For file back page */
if (page_mapping(page))
- extra += 1 + page_has_private(page);
+ extra += 1 + page_private_count(page);

if ((page_count(page) - extra) > page_mapcount(page))
return false;
diff --git a/mm/readahead.c b/mm/readahead.c
index f02dbebf1cef..661295ec4669 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -48,7 +48,7 @@ EXPORT_SYMBOL_GPL(file_ra_state_init);
static void read_cache_pages_invalidate_page(struct address_space *mapping,
struct page *page)
{
- if (page_has_private(page)) {
+ if (page_needs_cleanup(page)) {
if (!trylock_page(page))
BUG();
page->mapping = mapping;
diff --git a/mm/truncate.c b/mm/truncate.c
index 455944264663..7cad4c79686b 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -176,7 +176,7 @@ truncate_cleanup_page(struct address_space *mapping, struct page *page)
unmap_mapping_pages(mapping, page->index, nr, false);
}

- if (page_has_private(page))
+ if (page_needs_cleanup(page))
do_invalidatepage(page, 0, thp_size(page));

/*
@@ -204,7 +204,7 @@ invalidate_complete_page(struct address_space *mapping, struct page *page)
if (page->mapping != mapping)
return 0;

- if (page_has_private(page) && !try_to_release_page(page, 0))
+ if (page_needs_cleanup(page) && !try_to_release_page(page, 0))
return 0;

ret = remove_mapping(mapping, page);
@@ -346,7 +346,7 @@ void truncate_inode_pages_range(struct address_space *mapping,
wait_on_page_writeback(page);
zero_user_segment(page, partial_start, top);
cleancache_invalidate_page(mapping, page);
- if (page_has_private(page))
+ if (page_needs_cleanup(page))
do_invalidatepage(page, partial_start,
top - partial_start);
unlock_page(page);
@@ -359,7 +359,7 @@ void truncate_inode_pages_range(struct address_space *mapping,
wait_on_page_writeback(page);
zero_user_segment(page, 0, partial_end);
cleancache_invalidate_page(mapping, page);
- if (page_has_private(page))
+ if (page_needs_cleanup(page))
do_invalidatepage(page, 0,
partial_end);
unlock_page(page);
@@ -581,14 +581,14 @@ invalidate_complete_page2(struct address_space *mapping, struct page *page)
if (page->mapping != mapping)
return 0;

- if (page_has_private(page) && !try_to_release_page(page, GFP_KERNEL))
+ if (page_needs_cleanup(page) && !try_to_release_page(page, GFP_KERNEL))
return 0;

xa_lock_irqsave(&mapping->i_pages, flags);
if (PageDirty(page))
goto failed;

- BUG_ON(page_has_private(page));
+ BUG_ON(page_needs_cleanup(page));
__delete_from_page_cache(page, NULL);
xa_unlock_irqrestore(&mapping->i_pages, flags);

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 562e87cbd7a1..4d9928e3446d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -725,7 +725,7 @@ static inline int is_page_cache_freeable(struct page *page)
* heads at page->private.
*/
int page_cache_pins = thp_nr_pages(page);
- return page_count(page) - page_has_private(page) == 1 + page_cache_pins;
+ return page_count(page) - page_private_count(page) == 1 + page_cache_pins;
}

static int may_write_to_inode(struct inode *inode)
@@ -801,7 +801,7 @@ static pageout_t pageout(struct page *page, struct address_space *mapping)
* Some data journaling orphaned pages can have
* page->mapping == NULL while being dirty with clean buffers.
*/
- if (page_has_private(page)) {
+ if (page_needs_cleanup(page)) {
if (try_to_free_buffers(page)) {
ClearPageDirty(page);
pr_info("%s: orphaned page\n", __func__);
@@ -1057,7 +1057,7 @@ static void page_check_dirty_writeback(struct page *page,
*writeback = PageWriteback(page);

/* Verify dirty/writeback state if the filesystem supports it */
- if (!page_has_private(page))
+ if (!page_needs_cleanup(page))
return;

mapping = page_mapping(page);
@@ -1399,7 +1399,7 @@ static unsigned int shrink_page_list(struct list_head *page_list,
* 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 (page_needs_cleanup(page)) {
if (!try_to_release_page(page, sc->gfp_mask))
goto activate_locked;
if (!mapping && page_count(page) == 1) {
@@ -2050,8 +2050,8 @@ static void shrink_active_list(unsigned long nr_to_scan,
}

if (unlikely(buffer_heads_over_limit)) {
- if (page_has_private(page) && trylock_page(page)) {
- if (page_has_private(page))
+ if (page_needs_cleanup(page) && trylock_page(page)) {
+ if (page_needs_cleanup(page))
try_to_release_page(page, 0);
unlock_page(page);
}