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    SubjectRe: [PATCH] fix nr_unused accounting, and avoid recursing in iput with I_WILL_FREE set
    Andrea Arcangeli <andrea@suse.de> 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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