Andrea Arcangeli <[email protected]> wrote:
>
> Hello,
>
> @@ -183,6 +183,7 @@ __sync_single_inode(struct inode *inode,
> list_move(&inode->i_list, &inode_in_use);
> } else {
> list_move(&inode->i_list, &inode_unused);
> + inodes_stat.nr_unused++;
> }
> }
> wake_up_inode(inode);
>
> Are you sure the above diff is correct? It was added somewhere between
> 2.6.5 and 2.6.8. I think it's wrong.
>
> The only way I can imagine the i_count to be zero in the above path, is
> that I_WILL_FREE is set. And if I_WILL_FREE is set, then we must not
> increase nr_unused. So I believe the above change is buggy and it will
> definitely overstate the number of unused inodes and it should be backed
> out.
Well according to my assertion (below), the inode in __sync_single_inode()
cannot have a zero refcount, so the whole if() statement is never executed.
The thinking behind that increment is that __sync_single_inode() has just
taken a dirty, zero-refcount inode and has cleaned it. A dirty inode
cannot have previously been on inode_unused, hence we now are newly moving
it to inode_unused.
I'll stick a WARN_ON in there for now, wait and see if anyone can hit it.
> Note that __writeback_single_inode before calling __sync_single_inode, can
> drop the spinlock and we can have both the dirty and locked bitflags
> clear here:
>
> spin_unlock(&inode_lock);
> __wait_on_inode(inode);
> iput(inode);
> XXXXXXX
> spin_lock(&inode_lock);
> }
> use inode again here
>
> a construct like the above makes zero sense from a reference counting
> standpoint.
>
> Either we don't ever use the inode again after the iput, or the
> inode_lock should be taken _before_ executing the iput (i.e. a __iput
> would be required). Taking the inode_lock after iput means the iget was
> useless if we keep using the inode after the iput.
>
> So the only chance the 2.6 was safe to call __writeback_single_inode
> with the i_count == 0, is that I_WILL_FREE is set (I_WILL_FREE will
> prevent the VM to free the inode in XXXXX).
>
> Potentially calling the above iput with I_WILL_FREE was also wrong
> because it would recurse in iput_final (the second mainline bug).
>
> The below (untested) patch fixes the nr_unused accounting, avoids
> recursing in iput when I_WILL_FREE is set and makes sure (with the
> BUG_ON) that we don't corrupt memory and that all holders that don't set
> I_WILL_FREE, keeps a reference on the inode!
>
That's something which Bill snuck in there during some waitqueue rework.
I agree that the iget/iput is unneeded: all callers to
__writeback_single_inode() already have a ref on the inode: either via
sync_sb_inodes()'s iget() or via syscall(fd, ...).
So the BUG_ON() in __writeback_single_inode() becomes
BUG_ON(!atomic_read(&inode->i_count)); - I'll make it WARN_ON for now coz
I'm not so sure any more ;)
So... With updated comments:
diff -puN fs/fs-writeback.c~fix-nr_unused-accounting-and-avoid-recursing-in-iput-with-i_will_free-set fs/fs-writeback.c
--- 25/fs/fs-writeback.c~fix-nr_unused-accounting-and-avoid-recursing-in-iput-with-i_will_free-set Tue Oct 18 17:07:49 2005
+++ 25-akpm/fs/fs-writeback.c Tue Oct 18 17:12:53 2005
@@ -229,8 +229,8 @@ __sync_single_inode(struct inode *inode,
/*
* The inode is clean, unused
*/
+ WARN_ON(1);
list_move(&inode->i_list, &inode_unused);
- inodes_stat.nr_unused++;
}
}
wake_up_inode(inode);
@@ -238,14 +238,16 @@ __sync_single_inode(struct inode *inode,
}
/*
- * Write out an inode's dirty pages. Called under inode_lock.
+ * Write out an inode's dirty pages. Called under inode_lock. The caller has
+ * ref on the inode (either via __iget or via syscall against an fd).
*/
static int
-__writeback_single_inode(struct inode *inode,
- struct writeback_control *wbc)
+__writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
{
wait_queue_head_t *wqh;
+ WARN_ON(!atomic_read(&inode->i_count));
+
if ((wbc->sync_mode != WB_SYNC_ALL) && (inode->i_state & I_LOCK)) {
list_move(&inode->i_list, &inode->i_sb->s_dirty);
return 0;
@@ -259,11 +261,9 @@ __writeback_single_inode(struct inode *i
wqh = bit_waitqueue(&inode->i_state, __I_LOCK);
do {
- __iget(inode);
spin_unlock(&inode_lock);
__wait_on_bit(wqh, &wq, inode_wait,
TASK_UNINTERRUPTIBLE);
- iput(inode);
spin_lock(&inode_lock);
} while (inode->i_state & I_LOCK);
}
@@ -541,14 +541,15 @@ void sync_inodes(int wait)
}
/**
- * write_inode_now - write an inode to disk
- * @inode: inode to write to disk
- * @sync: whether the write should be synchronous or not
+ * write_inode_now - write an inode to disk
+ * @inode: inode to write to disk
+ * @sync: whether the write should be synchronous or not
+ *
+ * This function commits an inode to disk immediately if it is dirty. This is
+ * primarily needed by knfsd.
*
- * This function commits an inode to disk immediately if it is
- * dirty. This is primarily needed by knfsd.
+ * The caller must have a ref on the inode.
*/
-
int write_inode_now(struct inode *inode, int sync)
{
int ret;
_
-
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