ubifs: Use dirty_writeback_interval value for wbuf timer

Right now wbuf timer has hardcoded timeouts and there is no place for
manual adjustments. Some projects / cases many need that though. Few
file systems allow doing that by respecting dirty_writeback_interval
that can be set using sysctl (dirty_writeback_centisecs).

Lowering dirty_writeback_interval could be some way of dealing with user
space apps lacking proper fsyncs. This is definitely *not* a perfect
solution but we don't have ideal (user space) world. There were already
advanced discussions on this matter, mostly when ext4 was introduced and
it wasn't behaving as ext3. Anyway, the final decision was to add some
hacks to the ext4, as trying to fix whole user space or adding new API
was pointless.

We can't (and shouldn't?) just follow ext4. We can't e.g. sync on close
as this would cause too many commits and flash wearing. On the other
hand we still should allow some trade-off between -o sync and default
wbuf timeout. Respecting dirty_writeback_interval should allow some sane
cutomizations if used warily.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/io.c

@@ -452,11 +452,11 @@ static enum hrtimer_restart wbuf_timer_callback_nolock(struct hrtimer *timer)
  */
 static void new_wbuf_timer_nolock(struct ubifs_wbuf *wbuf)
 {
-	ktime_t softlimit = ktime_set(WBUF_TIMEOUT_SOFTLIMIT, 0);
-	unsigned long long delta;
+	ktime_t softlimit = ms_to_ktime(dirty_writeback_interval * 10);
+	unsigned long long delta = dirty_writeback_interval;
 
-	delta = WBUF_TIMEOUT_HARDLIMIT - WBUF_TIMEOUT_SOFTLIMIT;
-	delta *= 1000000000ULL;
+	/* centi to milli, milli to nano, then 10% */
+	delta *= 10ULL * NSEC_PER_MSEC / 10ULL;
 
 	ubifs_assert(!hrtimer_active(&wbuf->timer));
 	ubifs_assert(delta <= ULONG_MAX);

fs/ubifs/ubifs.h

@@ -83,10 +83,6 @@
  */
 #define BGT_NAME_PATTERN "ubifs_bgt%d_%d"
 
-/* Write-buffer synchronization timeout interval in seconds */
-#define WBUF_TIMEOUT_SOFTLIMIT 3
-#define WBUF_TIMEOUT_HARDLIMIT 5
-
 /* Maximum possible inode number (only 32-bit inodes are supported now) */
 #define MAX_INUM 0xFFFFFFFF