Syncthreads support.

This patch implements a new PF_SYNCTHREAD flag, which allows upcoming the refrigerator implementation to know that a thread is syncing data to disk. This allows the refrigerator to be more reliable, even under heavy load, because it can separate userspace processes that are submitting I/O from those trying to sync it and freezing the former first. We can thus ensure that syncing processes complete their work more quickly and the refrigerator is far less likely to incorrectly give up (thinking a syncing process is completely failing to enter the refrigerator).

Signed-off by: Nigel Cunningham <[email protected]>

 fs/buffer.c           |   45 +++++++++++++++++++++++++++++++++++++++++++--
 include/linux/sched.h |    2 ++
 2 files changed, 45 insertions(+), 2 deletions(-)

diff -ruNp 410-syncthreads.patch-old/fs/buffer.c 410-syncthreads.patch-new/fs/buffer.c

fs/buffer.c +43 -2

@@ -171,6 +171,15 @@ EXPORT_SYMBOL(sync_blockdev);
  */
 int fsync_super(struct super_block *sb)
 {
+	int ret;
+
+	/* A safety net. During suspend, we might overwrite
+	 * memory containing filesystem info. We don't then
+	 * want to sync it to disk. */
+	BUG_ON(test_suspend_state(SUSPEND_DISABLE_SYNCING));
+	
+	current->flags |= PF_SYNCTHREAD;
+
 	sync_inodes_sb(sb, 0);
 	DQUOT_SYNC(sb);
 	lock_super(sb);

@@ -182,7 +191,10 @@ int fsync_super(struct super_block *sb)
 	sync_blockdev(sb->s_bdev);
 	sync_inodes_sb(sb, 1);
 
-	return sync_blockdev(sb->s_bdev);
+	ret = sync_blockdev(sb->s_bdev);
+
+	current->flags &= ~PF_SYNCTHREAD;
+	return ret;
 }
 
 /*

@@ -193,12 +205,21 @@ int fsync_super(struct super_block *sb)
 int fsync_bdev(struct block_device *bdev)
 {
 	struct super_block *sb = get_super(bdev);
+	int ret;
+
+	BUG_ON(test_suspend_state(SUSPEND_DISABLE_SYNCING));
+
+	current->flags |= PF_SYNCTHREAD;
+
 	if (sb) {
 		int res = fsync_super(sb);
 		drop_super(sb);
+		current->flags &= ~PF_SYNCTHREAD;
 		return res;
 	}
-	return sync_blockdev(bdev);
+	ret = sync_blockdev(bdev);
+	current->flags &= ~PF_SYNCTHREAD;
+	return ret;
 }
 
 /**

@@ -278,6 +299,13 @@ EXPORT_SYMBOL(thaw_bdev);
  */
 static void do_sync(unsigned long wait)
 {
+	/* A safety net. During suspend, we might overwrite
+	 * memory containing filesystem info. We don't then
+	 * want to sync it to disk. */
+	BUG_ON(test_suspend_state(SUSPEND_DISABLE_SYNCING));
+
+	current->flags |= PF_SYNCTHREAD;
+
 	wakeup_pdflush(0);
 	sync_inodes(0);		/* All mappings, inodes and their blockdevs */
 	DQUOT_SYNC(NULL);

@@ -289,6 +317,8 @@ static void do_sync(unsigned long wait)
 		printk("Emergency Sync complete\n");
 	if (unlikely(laptop_mode))
 		laptop_sync_completion();
+
+	current->flags &= ~PF_SYNCTHREAD;
 }
 
 asmlinkage long sys_sync(void)

@@ -314,6 +344,10 @@ int file_fsync(struct file *filp, struct
 	struct super_block * sb;
 	int ret, err;
 
+	BUG_ON(test_suspend_state(SUSPEND_DISABLE_SYNCING));
+
+	current->flags |= PF_SYNCTHREAD;
+
 	/* sync the inode to buffers */
 	ret = write_inode_now(inode, 0);
 

@@ -328,6 +362,8 @@ int file_fsync(struct file *filp, struct
 	err = sync_blockdev(sb->s_bdev);
 	if (!ret)
 		ret = err;
+
+	current->flags &= ~PF_SYNCTHREAD;
 	return ret;
 }
 

@@ -337,6 +373,10 @@ static long do_fsync(unsigned int fd, in
 	struct address_space *mapping;
 	int ret, err;
 
+	BUG_ON(test_suspend_state(SUSPEND_DISABLE_SYNCING));
+
+	current->flags |= PF_SYNCTHREAD;
+
 	ret = -EBADF;
 	file = fget(fd);
 	if (!file)

@@ -370,6 +410,7 @@ static long do_fsync(unsigned int fd, in
 out_putf:
 	fput(file);
 out:
+	current->flags &= ~PF_SYNCTHREAD;
 	return ret;
 }
 

diff -ruNp 410-syncthreads.patch-old/include/linux/sched.h 410-syncthreads.patch-new/include/linux/sched.h

include/linux/sched.h +2 -0

@@ -829,6 +829,8 @@ do { if (atomic_dec_and_test(&(tsk)->usa
 #define PF_SYNCWRITE	0x00200000	/* I am doing a sync write */
 #define PF_BORROWED_MM	0x00400000	/* I am a kthread doing use_mm */
 #define PF_RANDOMIZE	0x00800000	/* randomize virtual address space */
+#define PF_SYNCTHREAD	0x01000000	/* this thread can start activity during the 
++ 					   early part of freezing processes */
 
 /*
  * Only the _current_ task can read/write to tsk->flags, but other

Hi!

Quoted 8 lines

This patch implements a new PF_SYNCTHREAD flag, which allows upcoming the refrigerator implementation to know that a thread is syncing data to disk. This allows the refrigerator to be more reliable, even under heavy load, because it can separate userspace processes that are submitting I/O from those trying to sync it and freezing the former first. We can thus ensure that syncing processes complete their work more quickly and the refrigerator is far less likely to incorrectly give up (thinking a syncing process is completely failing to enter the refrigerator).

This patch is ****, and pretty intrusive too. Ouch and then it is unneccessary. We have been over this before, and explained to you in person. Greg explained it to you, too. This one is NOT GOING IN. Drop it from your patches, trying to submit it 10 times will not get it accepted.

[For the record, simple solution for this one is

sys_sync();

and only then start refrigeration].

This patch implements a new PF_SYNCTHREAD flag, which allows upcoming the refrigerator implementation to know that a thread is syncing data to disk. This allows the refrigerator to be more reliable, even under heavy load, because it can separate userspace processes that are submitting I/O from those trying to sync it and freezing the former first. We can thus ensure that syncing processes complete their work more quickly and the refrigerator is far less likely to incorrectly give up (thinking a syncing process is completely failing to enter the refrigerator).

Quoted 8 lines · Nigel Cunningham

This patch implements a new PF_SYNCTHREAD flag, which allows upcoming the refrigerator implementation to know that a thread is syncing data to disk. This allows the refrigerator to be more reliable, even under heavy load, because it can separate userspace processes that are submitting I/O from those trying to sync it and freezing the former first. We can thus ensure that syncing processes complete their work more quickly and the refrigerator is far less likely to incorrectly give up (thinking a syncing process is completely failing to enter the refrigerator).

I don't have any strong feelings for this, one way or another. It seems kinda hacky, and it needs to be discussed publically, and it seems like it definitely seems like it doesn't need to go in immediately.

I have just a couple of suggestions below..

fs/buffer.c EXPORT_SYMBOL(sync_blockdev); · line 179

Quoted 8 lines

int fsync_super(struct super_block *sb)
{

fs/buffer.c +6 -0

+	int ret;
+
+	/* A safety net. During suspend, we might overwrite
+	 * memory containing filesystem info. We don't then
+	 * want to sync it to disk. */
+	BUG_ON(test_suspend_state(SUSPEND_DISABLE_SYNCING));

Please do not add any new BUG()s. Figure out another way to gracefully fail, perhaps some place else.

Review

Quoted 1 line

(quoted diff) +1 -0

+	current->flags |= PF_SYNCTHREAD;

Is all the modification of current->flags safe? It seems like you could be pre-empted in the middle and things could get wacky.

Another note is that these changes will have to go through Al Viro, who might have some suggestions on the Right(tm) way to do it.