[PATCH v2 23/26] btrfs: teach __extent_writepage() to handle locked page differently

Qu Wenruo
2021-08-29T05:25:34Z

Pages passed to __extent_writepage() are always locked, but they may be
locked by different functions.

There are two types of locked page for __extent_writepage():

- Page locked by plain lock_page()
It should not have any subpage::writers count.
Can be unlocked by unlock_page().
This is the most common locked page for __extent_writepage() called
inside extent_write_cache_pages() or extent_write_full_page().
Rarer cases includes the @locked_page from extent_write_locked_range().

- Page locked by lock_delalloc_pages()
There is only one caller, all pages except @locked_page for
extent_write_locked_range().
In this case, we have to call subpage helper to handle the case.

So here we introduce a helper, btrfs_page_unlock_writer(), to allow
__extent_writepage() to unlock different locked pages.

And since for all other callers of __extent_writepage() their pages are
ensured to be locked by lock_page(), also add an extra check for
epd::extent_locked to unlock such pages directly.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
fs/btrfs/extent_io.c | 15 ++++++++++++++-
fs/btrfs/subpage.c | 43 +++++++++++++++++++++++++++++++++++++++++++
fs/btrfs/subpage.h | 2 ++
3 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 077856bf15d0..44df7e142fb7 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -4060,6 +4060,7 @@ static int __extent_writepage(struct page *page, struct writeback_control *wbc,
struct extent_page_data *epd)
{
struct inode *inode = page->mapping->host;
+ struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
const u64 page_start = page_offset(page);
const u64 page_end = page_start + PAGE_SIZE - 1;
int ret;
@@ -4148,7 +4149,19 @@ static int __extent_writepage(struct page *page, struct writeback_control *wbc,
*/
if (PageError(page))
end_extent_writepage(page, ret, page_start, page_end);
- unlock_page(page);
+ if (epd->extent_locked) {
+ /*
+ * If epd->extent_locked, it's from extent_write_locked_range(),
+ * the page can either be locked by lock_page() or
+ * process_one_page().
+ * Let btrfs_page_unlock_writer() to handle both cases.
+ */
+ ASSERT(wbc);
+ btrfs_page_unlock_writer(fs_info, page, wbc->range_start,
+ wbc->range_end + 1 - wbc->range_start);
+ } else {
+ unlock_page(page);
+ }
ASSERT(ret <= 0);
return ret;
}
diff --git a/fs/btrfs/subpage.c b/fs/btrfs/subpage.c
index 2bea6766d84e..50b3d96289ca 100644
--- a/fs/btrfs/subpage.c
+++ b/fs/btrfs/subpage.c
@@ -691,3 +691,46 @@ void btrfs_page_assert_not_dirty(const struct btrfs_fs_info *fs_info,
ASSERT(PagePrivate(page) && page->private);
ASSERT(subpage_test_bitmap_all_zero(fs_info, subpage, dirty));
}
+
+/*
+ * Helper to handle different locked page with different page size
+ * - Page locked by plain lock_page()
+ * It should not have any subpage::writers count.
+ * Can be unlocked by unlock_page().
+ * This is the most common locked page for __extent_writepage() called
+ * inside extent_write_cache_pages() or extent_write_full_page().
+ * Rarer cases includes the @locked_page from extent_write_locked_range().
+ *
+ * - Page locked by lock_delalloc_pages()
+ * There is only one caller, all pages except @locked_page for
+ * extent_write_locked_range().
+ * In this case, we have to call subpage helper to handle the case.
+ */
+void btrfs_page_unlock_writer(struct btrfs_fs_info *fs_info, struct page *page,
+ u64 start, u32 len)
+{
+ struct btrfs_subpage *subpage;
+
+ ASSERT(PageLocked(page));
+ /* For regular page size case, we just unlock the page */
+ if (fs_info->sectorsize == PAGE_SIZE)
+ return unlock_page(page);
+
+ ASSERT(PagePrivate(page) && page->private);
+ subpage = (struct btrfs_subpage *)page->private;
+
+ /*
+ * For subpage case, there are two types of locked page.
+ * With or without writers number.
+ *
+ * Since we own the page lock, no one else could touch
+ * subpage::writers and are we safe to do several atomic operations
+ * without spinlock.
+ */
+ if (atomic_read(&subpage->writers))
+ /* No writers, locked by plain lock_page() */
+ return unlock_page(page);
+
+ /* Have writers, use proper subpage helper to end it */
+ btrfs_page_end_writer_lock(fs_info, page, start, len);
+}
diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
index 46224f959c34..7accb5c40d33 100644
--- a/fs/btrfs/subpage.h
+++ b/fs/btrfs/subpage.h
@@ -150,5 +150,7 @@ bool btrfs_subpage_clear_and_test_dirty(const struct btrfs_fs_info *fs_info,

void btrfs_page_assert_not_dirty(const struct btrfs_fs_info *fs_info,
struct page *page);
+void btrfs_page_unlock_writer(struct btrfs_fs_info *fs_info, struct page *page,
+ u64 start, u32 len);

#endif
--
2.32.0