[PATCH] f2fs: don't over-report free space or inodes in statvfs

[Chao Yu <chao@kernel.org>]

This fixes an analogus bug that was fixed in modern filesystems:
a) xfs in commit 4b8d867ca6e2 ("xfs: don't over-report free space or
inodes in statvfs")
b) ext4 in commit f87d3af74193 ("ext4: don't over-report free space
or inodes in statvfs")
where statfs can report misleading / incorrect information where
project quota is enabled, and the free space is less than the
remaining quota.

This commit will resolve a test failure in generic/762 which tests
for this bug.

generic/762       - output mismatch (see /share/git/fstests/results//generic/762.out.bad)
    --- tests/generic/762.out   2025-04-15 10:21:53.371067071 +0800
    +++ /share/git/fstests/results//generic/762.out.bad 2025-05-13 16:13:37.000000000 +0800
    @@ -6,8 +6,10 @@
     root blocks2 is in range
     dir blocks2 is in range
     root bavail2 is in range
    -dir bavail2 is in range
    +dir bavail2 has value of 1539066
    +dir bavail2 is NOT in range 304734.87 .. 310891.13
     root blocks3 is in range
    ...
    (Run 'diff -u /share/git/fstests/tests/generic/762.out /share/git/fstests/results//generic/762.out.bad'  to see the entire diff)

HINT: You _MAY_ be missing kernel fix:
      XXXXXXXXXXXXXX xfs: don't over-report free space or inodes in statvfs

Cc: stable@kernel.org
Fixes: ddc34e328d06 ("f2fs: introduce f2fs_statfs_project")
Signed-off-by: Chao Yu <chao@kernel.org>
---
 fs/f2fs/super.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 32f2abac19cf..da64760af74b 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -2069,26 +2069,32 @@ static int f2fs_statfs_project(struct super_block *sb,
 
 	limit = min_not_zero(dquot->dq_dqb.dqb_bsoftlimit,
 					dquot->dq_dqb.dqb_bhardlimit);
-	if (limit)
-		limit >>= sb->s_blocksize_bits;
+	limit >>= sb->s_blocksize_bits;
+
+	if (limit) {
+		uint64_t remaining = 0;
 
-	if (limit && buf->f_blocks > limit) {
 		curblock = (dquot->dq_dqb.dqb_curspace +
 			    dquot->dq_dqb.dqb_rsvspace) >> sb->s_blocksize_bits;
-		buf->f_blocks = limit;
-		buf->f_bfree = buf->f_bavail =
-			(buf->f_blocks > curblock) ?
-			 (buf->f_blocks - curblock) : 0;
+		if (limit > curblock)
+			remaining = limit - curblock;
+
+		buf->f_blocks = min(buf->f_blocks, limit);
+		buf->f_bfree = min(buf->f_bfree, remaining);
+		buf->f_bavail = min(buf->f_bavail, remaining);
 	}
 
 	limit = min_not_zero(dquot->dq_dqb.dqb_isoftlimit,
 					dquot->dq_dqb.dqb_ihardlimit);
 
-	if (limit && buf->f_files > limit) {
-		buf->f_files = limit;
-		buf->f_ffree =
-			(buf->f_files > dquot->dq_dqb.dqb_curinodes) ?
-			 (buf->f_files - dquot->dq_dqb.dqb_curinodes) : 0;
+	if (limit) {
+		uint64_t remaining = 0;
+
+		if (limit > dquot->dq_dqb.dqb_curinodes)
+			remaining = limit - dquot->dq_dqb.dqb_curinodes;
+
+		buf->f_files = min(buf->f_files, limit);
+		buf->f_ffree = min(buf->f_ffree, remaining);
 	}
 
 	spin_unlock(&dquot->dq_dqb_lock);
-- 
2.49.0