[RFC PATCH v4 02/12] fs: Add counter to track inflight writes that need stable pages

[Ojaswin Mujoo <ojaswin@linux.ibm.com>]

The current flag-style stable write implementation uses idempotent set
and clear functions. This is okay because the users for the most part
just want to set or clear it once based on factors like underlying
device support.

However, this scheme doesn't play well when we have parallel users
wanting to temporarily set and unset stable writes. For example, the
upcoming RWF_WRITETHROUGH patches need stable writes to be enabled for
the duration of the IO. The current scheme can lead to bugs like:

RWF_WRITETHROUGH write 1             RWF_WRITETHROUGH write 2

enable stable write		     enable stable write
submit IO
disable stable write   <---- WRONG

                                     submit IO
                                     disable stable write

The 2nd write loses the stable write guarantee midway which is not
correct. Fix this by introducing a new inflight_stable_write counter
which can be used by parallel users safely.

Unfortunately, due to the way the current users are designed, we cannot
directly migrate them to the counter approach hence for now we will
have to keep both methods till all the users adapt to the counters.

Suggested-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
---
 include/linux/fs.h      |  1 +
 include/linux/pagemap.h | 14 +++++++++++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 05d05bb9c951..f196f25255ab 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -482,6 +482,7 @@ struct address_space {
 	errseq_t		wb_err;
 	spinlock_t		i_private_lock;
 	struct rw_semaphore	i_mmap_rwsem;
+	atomic_t		inflight_stable_writes_count;
 } __attribute__((aligned(sizeof(long)))) __randomize_layout;
 	/*
 	 * On most architectures that alignment is already the case; but
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 0adfa6605653..59713769f731 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -306,7 +306,8 @@ static inline void mapping_clear_release_always(struct address_space *mapping)
 
 static inline bool mapping_stable_writes(const struct address_space *mapping)
 {
-	return test_bit(AS_STABLE_WRITES, &mapping->flags);
+	return test_bit(AS_STABLE_WRITES, &mapping->flags) ||
+	       atomic_read(&mapping->inflight_stable_writes_count) > 0;
 }
 
 static inline void mapping_set_stable_writes(struct address_space *mapping)
@@ -319,6 +320,17 @@ static inline void mapping_clear_stable_writes(struct address_space *mapping)
 	clear_bit(AS_STABLE_WRITES, &mapping->flags);
 }
 
+static inline void mapping_inc_inflight_stable_writes(struct address_space *mapping)
+{
+	atomic_inc(&mapping->inflight_stable_writes_count);
+}
+
+static inline void mapping_dec_inflight_stable_writes(struct address_space *mapping)
+{
+	WARN_ON_ONCE(atomic_dec_if_positive(
+			     &mapping->inflight_stable_writes_count) < 0);
+}
+
 static inline void mapping_set_inaccessible(struct address_space *mapping)
 {
 	/*
-- 
2.55.0