[1/3] writeback: Avoid skipping inode writeback
diff mbox series

Message ID 20200601091904.4786-1-jack@suse.cz
State New
Headers show
Series
  • writeback: Lazytime handling fix and cleanups
Related show

Commit Message

Jan Kara June 1, 2020, 9:18 a.m. UTC
Inode's i_io_list list head is used to attach inode to several different
lists - wb->{b_dirty, b_dirty_time, b_io, b_more_io}. When flush worker
prepares a list of inodes to writeback e.g. for sync(2), it moves inodes
to b_io list. Thus it is critical for sync(2) data integrity guarantees
that inode is not requeued to any other writeback list when inode is
queued for processing by flush worker. That's the reason why
writeback_single_inode() does not touch i_io_list (unless the inode is
completely clean) and why __mark_inode_dirty() does not touch i_io_list
if I_SYNC flag is set.

However there are two flaws in the current logic:

1) When inode has only I_DIRTY_TIME set but it is already queued in b_io
list due to sync(2), concurrent __mark_inode_dirty(inode, I_DIRTY_SYNC)
can still move inode back to b_dirty list resulting in skipping
writeback of inode time stamps during sync(2).

2) When inode is on b_dirty_time list and writeback_single_inode() races
with __mark_inode_dirty() like:

writeback_single_inode()		__mark_inode_dirty(inode, I_DIRTY_PAGES)
  inode->i_state |= I_SYNC
  __writeback_single_inode()
					  inode->i_state |= I_DIRTY_PAGES;
					  if (inode->i_state & I_SYNC)
					    bail
  if (!(inode->i_state & I_DIRTY_ALL))
  - not true so nothing done

We end up with I_DIRTY_PAGES inode on b_dirty_time list and thus
standard background writeback will not writeback this inode leading to
possible dirty throttling stalls etc. (thanks to Martijn Coenen for this
analysis).

Fix these problems by tracking whether inode is queued in b_io or
b_more_io lists in a new I_SYNC_QUEUED flag. When this flag is set, we
know flush worker has queued inode and we should not touch i_io_list.
On the other hand we also know that once flush worker is done with the
inode it will requeue the inode to appropriate dirty list. When
I_SYNC_QUEUED is not set, __mark_inode_dirty() can (and must) move inode
to appropriate dirty list.

Reported-by: Martijn Coenen <maco@android.com>
Fixes: 0ae45f63d4ef ("vfs: add support for a lazytime mount option")
CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/fs-writeback.c  | 39 +++++++++++++++++++++++++++++----------
 include/linux/fs.h |  8 ++++++--
 2 files changed, 35 insertions(+), 12 deletions(-)

Comments

Sasha Levin June 5, 2020, 2:11 p.m. UTC | #1
Hi

[This is an automated email]

This commit has been processed because it contains a "Fixes:" tag
fixing commit: 0ae45f63d4ef ("vfs: add support for a lazytime mount option").

The bot has tested the following trees: v5.6.15, v5.4.43, v4.19.125, v4.14.182, v4.9.225, v4.4.225.

v5.6.15: Build OK!
v5.4.43: Build OK!
v4.19.125: Build OK!
v4.14.182: Failed to apply! Possible dependencies:
    80ea09a002bf ("vfs: factor out inode_insert5()")
    c2b6d621c4ff ("new primitive: discard_new_inode()")

v4.9.225: Failed to apply! Possible dependencies:
    09d8b586731b ("ovl: move __upperdentry to ovl_inode")
    13c72075ac9f ("ovl: move impure to ovl_inode")
    2aff4534b6c4 ("ovl: check lower existence when removing")
    2b8c30e9ef14 ("ovl: use d_is_dir()")
    32a3d848eb91 ("ovl: clean up kstat usage")
    370e55ace59c ("ovl: rename: simplify handling of lower/merged directory")
    38e813db61c3 ("ovl: get rid of PURE type")
    3ee23ff1025a ("ovl: check lower existence of rename target")
    42f269b92540 ("ovl: rearrange code in ovl_copy_up_locked()")
    5cf5b477f0ca ("ovl: opaque cleanup")
    6c02cb59e6fe ("ovl: rename ovl_rename2() to ovl_rename()")
    804032fabb3b ("ovl: don't check rename to self")
    80ea09a002bf ("vfs: factor out inode_insert5()")
    8ee6059c58ea ("ovl: simplify lookup")
    a6c606551141 ("ovl: redirect on rename-dir")
    ad0af7104dad ("vfs: introduce inode 'inuse' lock")
    bbb1e54dd53c ("ovl: split super.c")
    c2b6d621c4ff ("new primitive: discard_new_inode()")
    c412ce498396 ("ovl: add ovl_dentry_is_whiteout()")
    ca4c8a3a8000 ("ovl: treat special files like a regular fs")
    d8514d8edb5b ("ovl: copy up regular file using O_TMPFILE")

v4.4.225: Failed to apply! Possible dependencies:
    09d8b586731b ("ovl: move __upperdentry to ovl_inode")
    1175b6b8d963 ("ovl: do operations on underlying file system in mounter's context")
    13c72075ac9f ("ovl: move impure to ovl_inode")
    2864f3014242 ("iget_locked et.al.: make sure we don't return bad inodes")
    32a3d848eb91 ("ovl: clean up kstat usage")
    38b256973ea9 ("ovl: handle umask and posix_acl_default correctly on creation")
    42f269b92540 ("ovl: rearrange code in ovl_copy_up_locked()")
    6b2553918d8b ("replace ->follow_link() with new method that could stay in RCU mode")
    80ea09a002bf ("vfs: factor out inode_insert5()")
    aa80deab33a8 ("namei: page_getlink() and page_follow_link_light() are the same thing")
    ad0af7104dad ("vfs: introduce inode 'inuse' lock")
    bb0d2b8ad296 ("ovl: fix sgid on directory")
    c2b6d621c4ff ("new primitive: discard_new_inode()")
    d6335d77a762 ("security: Make inode argument of inode_getsecid non-const")
    d8514d8edb5b ("ovl: copy up regular file using O_TMPFILE")
    d8ad8b496184 ("security, overlayfs: provide copy up security hook for unioned files")
    fceef393a538 ("switch ->get_link() to delayed_call, kill ->put_link()")


NOTE: The patch will not be queued to stable trees until it is upstream.

How should we proceed with this patch?
Christoph Hellwig June 10, 2020, 3:02 p.m. UTC | #2
This generall looks ok, but a few nitpicks below:

> -static void redirty_tail(struct inode *inode, struct bdi_writeback *wb)
> +static void __redirty_tail(struct inode *inode, struct bdi_writeback *wb)

I think redirty_tail_locked would be a more decriptive name, and also
fit other uses in this file (e.g. inode_io_list_move_locked and
inode_io_list_del_locked).

>  {
> +	assert_spin_locked(&inode->i_lock);
>  	if (!list_empty(&wb->b_dirty)) {

Nit: I find an empty line after asserts and before the real code starts
nice on the eye.

>  			break;
>  		list_move(&inode->i_io_list, &tmp);
>  		moved++;
> +		spin_lock(&inode->i_lock);
>  		if (flags & EXPIRE_DIRTY_ATIME)
> -			set_bit(__I_DIRTY_TIME_EXPIRED, &inode->i_state);
> +			inode->i_state |= I_DIRTY_TIME_EXPIRED;
> +		inode->i_state |= I_SYNC_QUEUED;
> +		spin_unlock(&inode->i_lock);

I wonder if the locking changes should go into a prep patch vs the
actual logic changes related to I_SYNC_QUEUED?  That would untangle
the patch quite a bit and make it easier to follow.

>  #define I_WB_SWITCH		(1 << 13)
>  #define I_OVL_INUSE		(1 << 14)
>  #define I_CREATING		(1 << 15)
> +#define I_SYNC_QUEUED		(1 << 16)

FYI, this conflicts with the I_DONTCAT addition in mainline now.
Jan Kara June 10, 2020, 3:30 p.m. UTC | #3
On Wed 10-06-20 08:02:03, Christoph Hellwig wrote:
> This generall looks ok, but a few nitpicks below:
> 
> > -static void redirty_tail(struct inode *inode, struct bdi_writeback *wb)
> > +static void __redirty_tail(struct inode *inode, struct bdi_writeback *wb)
> 
> I think redirty_tail_locked would be a more decriptive name, and also
> fit other uses in this file (e.g. inode_io_list_move_locked and
> inode_io_list_del_locked).

Fair enough, will do.

> >  {
> > +	assert_spin_locked(&inode->i_lock);
> >  	if (!list_empty(&wb->b_dirty)) {
> 
> Nit: I find an empty line after asserts and before the real code starts
> nice on the eye.

Sure.

> >  			break;
> >  		list_move(&inode->i_io_list, &tmp);
> >  		moved++;
> > +		spin_lock(&inode->i_lock);
> >  		if (flags & EXPIRE_DIRTY_ATIME)
> > -			set_bit(__I_DIRTY_TIME_EXPIRED, &inode->i_state);
> > +			inode->i_state |= I_DIRTY_TIME_EXPIRED;
> > +		inode->i_state |= I_SYNC_QUEUED;
> > +		spin_unlock(&inode->i_lock);
> 
> I wonder if the locking changes should go into a prep patch vs the
> actual logic changes related to I_SYNC_QUEUED?  That would untangle
> the patch quite a bit and make it easier to follow.

OK, will do.
 
> >  #define I_WB_SWITCH		(1 << 13)
> >  #define I_OVL_INUSE		(1 << 14)
> >  #define I_CREATING		(1 << 15)
> > +#define I_SYNC_QUEUED		(1 << 16)
> 
> FYI, this conflicts with the I_DONTCAT addition in mainline now.

Yup, I've already found out when rebasing...

Thanks for review!

								Honza

Patch
diff mbox series

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 76ac9c7d32ec..855c6611710a 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -144,7 +144,9 @@  static void inode_io_list_del_locked(struct inode *inode,
 				     struct bdi_writeback *wb)
 {
 	assert_spin_locked(&wb->list_lock);
+	assert_spin_locked(&inode->i_lock);
 
+	inode->i_state &= ~I_SYNC_QUEUED;
 	list_del_init(&inode->i_io_list);
 	wb_io_lists_depopulated(wb);
 }
@@ -1123,7 +1125,9 @@  void inode_io_list_del(struct inode *inode)
 	struct bdi_writeback *wb;
 
 	wb = inode_to_wb_and_lock_list(inode);
+	spin_lock(&inode->i_lock);
 	inode_io_list_del_locked(inode, wb);
+	spin_unlock(&inode->i_lock);
 	spin_unlock(&wb->list_lock);
 }
 
@@ -1172,8 +1176,9 @@  void sb_clear_inode_writeback(struct inode *inode)
  * the case then the inode must have been redirtied while it was being written
  * out and we don't reset its dirtied_when.
  */
-static void redirty_tail(struct inode *inode, struct bdi_writeback *wb)
+static void __redirty_tail(struct inode *inode, struct bdi_writeback *wb)
 {
+	assert_spin_locked(&inode->i_lock);
 	if (!list_empty(&wb->b_dirty)) {
 		struct inode *tail;
 
@@ -1182,6 +1187,14 @@  static void redirty_tail(struct inode *inode, struct bdi_writeback *wb)
 			inode->dirtied_when = jiffies;
 	}
 	inode_io_list_move_locked(inode, wb, &wb->b_dirty);
+	inode->i_state &= ~I_SYNC_QUEUED;
+}
+
+static void redirty_tail(struct inode *inode, struct bdi_writeback *wb)
+{
+	spin_lock(&inode->i_lock);
+	__redirty_tail(inode, wb);
+	spin_unlock(&inode->i_lock);
 }
 
 /*
@@ -1250,8 +1263,11 @@  static int move_expired_inodes(struct list_head *delaying_queue,
 			break;
 		list_move(&inode->i_io_list, &tmp);
 		moved++;
+		spin_lock(&inode->i_lock);
 		if (flags & EXPIRE_DIRTY_ATIME)
-			set_bit(__I_DIRTY_TIME_EXPIRED, &inode->i_state);
+			inode->i_state |= I_DIRTY_TIME_EXPIRED;
+		inode->i_state |= I_SYNC_QUEUED;
+		spin_unlock(&inode->i_lock);
 		if (sb_is_blkdev_sb(inode->i_sb))
 			continue;
 		if (sb && sb != inode->i_sb)
@@ -1394,7 +1410,7 @@  static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
 		 * writeback is not making progress due to locked
 		 * buffers. Skip this inode for now.
 		 */
-		redirty_tail(inode, wb);
+		__redirty_tail(inode, wb);
 		return;
 	}
 
@@ -1414,7 +1430,7 @@  static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
 			 * retrying writeback of the dirty page/inode
 			 * that cannot be performed immediately.
 			 */
-			redirty_tail(inode, wb);
+			__redirty_tail(inode, wb);
 		}
 	} else if (inode->i_state & I_DIRTY) {
 		/*
@@ -1422,10 +1438,11 @@  static void requeue_inode(struct inode *inode, struct bdi_writeback *wb,
 		 * such as delayed allocation during submission or metadata
 		 * updates after data IO completion.
 		 */
-		redirty_tail(inode, wb);
+		__redirty_tail(inode, wb);
 	} else if (inode->i_state & I_DIRTY_TIME) {
 		inode->dirtied_when = jiffies;
 		inode_io_list_move_locked(inode, wb, &wb->b_dirty_time);
+		inode->i_state &= ~I_SYNC_QUEUED;
 	} else {
 		/* The inode is clean. Remove from writeback lists. */
 		inode_io_list_del_locked(inode, wb);
@@ -1669,8 +1686,9 @@  static long writeback_sb_inodes(struct super_block *sb,
 		 */
 		spin_lock(&inode->i_lock);
 		if (inode->i_state & (I_NEW | I_FREEING | I_WILL_FREE)) {
+			inode->i_state &= ~I_SYNC_QUEUED;
+			__redirty_tail(inode, wb);
 			spin_unlock(&inode->i_lock);
-			redirty_tail(inode, wb);
 			continue;
 		}
 		if ((inode->i_state & I_SYNC) && wbc.sync_mode != WB_SYNC_ALL) {
@@ -2289,11 +2307,12 @@  void __mark_inode_dirty(struct inode *inode, int flags)
 		inode->i_state |= flags;
 
 		/*
-		 * If the inode is being synced, just update its dirty state.
-		 * The unlocker will place the inode on the appropriate
-		 * superblock list, based upon its state.
+		 * If the inode is queued for writeback by flush worker, just
+		 * update its dirty state. Once the flush worker is done with
+		 * the inode it will place it on the appropriate superblock
+		 * list, based upon its state.
 		 */
-		if (inode->i_state & I_SYNC)
+		if (inode->i_state & I_SYNC_QUEUED)
 			goto out_unlock_inode;
 
 		/*
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 45cc10cdf6dd..b02290d19edd 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2156,6 +2156,10 @@  static inline void kiocb_clone(struct kiocb *kiocb, struct kiocb *kiocb_src,
  *
  * I_CREATING		New object's inode in the middle of setting up.
  *
+ * I_SYNC_QUEUED	Inode is queued in b_io or b_more_io writeback lists.
+ *			Used to detect that mark_inode_dirty() should not move
+ * 			inode between dirty lists.
+ *
  * Q: What is the difference between I_WILL_FREE and I_FREEING?
  */
 #define I_DIRTY_SYNC		(1 << 0)
@@ -2173,11 +2177,11 @@  static inline void kiocb_clone(struct kiocb *kiocb, struct kiocb *kiocb_src,
 #define I_DIO_WAKEUP		(1 << __I_DIO_WAKEUP)
 #define I_LINKABLE		(1 << 10)
 #define I_DIRTY_TIME		(1 << 11)
-#define __I_DIRTY_TIME_EXPIRED	12
-#define I_DIRTY_TIME_EXPIRED	(1 << __I_DIRTY_TIME_EXPIRED)
+#define I_DIRTY_TIME_EXPIRED	(1 << 12)
 #define I_WB_SWITCH		(1 << 13)
 #define I_OVL_INUSE		(1 << 14)
 #define I_CREATING		(1 << 15)
+#define I_SYNC_QUEUED		(1 << 16)
 
 #define I_DIRTY_INODE (I_DIRTY_SYNC | I_DIRTY_DATASYNC)
 #define I_DIRTY (I_DIRTY_INODE | I_DIRTY_PAGES)