f2fs: Accounting large folio subpages before bio submission

In f2fs_read_data_large_folio(), read_pages_pending is incremented only
after the subpage has been added to the BIO.  With a heavily fragmented
file, each new subpage can force submission of the previous BIO.

If the BIO completes quickly, f2fs_finish_read_bio() may decrement
read_pages_pending to zero and call folio_end_read() while the read loop
is still processing other subpages of the same large folio.

Fix the ordering by incrementing read_pages_pending before any possible
BIO submission for the current subpage, matching the iomap ordering and
preventing premature folio_end_read().

Signed-off-by: Nanzhe Zhao <nzzhao@126.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: sigmanature

fs/f2fs/data.c

@@ -2497,6 +2497,18 @@ static int f2fs_read_data_large_folio(struct inode *inode,
 			continue;
 		}
 
+		/* We must increment read_pages_pending before possible BIOs submitting
+		 * to prevent from premature folio_end_read() call on folio
+		 */
+		if (folio_test_large(folio)) {
+			ffs = ffs_find_or_alloc(folio);
+
+			/* set the bitmap to wait */
+			spin_lock_irq(&ffs->state_lock);
+			ffs->read_pages_pending++;
+			spin_unlock_irq(&ffs->state_lock);
+		}
+
 		/*
 		 * This page will go to BIO.  Do we need to send this
 		 * BIO off first?
@@ -2524,15 +2536,6 @@ static int f2fs_read_data_large_folio(struct inode *inode,
 					offset << PAGE_SHIFT))
 			goto submit_and_realloc;
 
-		if (folio_test_large(folio)) {
-			ffs = ffs_find_or_alloc(folio);
-
-			/* set the bitmap to wait */
-			spin_lock_irq(&ffs->state_lock);
-			ffs->read_pages_pending++;
-			spin_unlock_irq(&ffs->state_lock);
-		}
-
 		inc_page_count(F2FS_I_SB(inode), F2FS_RD_DATA);
 		f2fs_update_iostat(F2FS_I_SB(inode), NULL, FS_DATA_READ_IO,
 				F2FS_BLKSIZE);