linux-kernel - Re: The bug of iput() removal from flusher thread?

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Message-ID: <20121119212448.GA29498@quack.suse.cz>
Date:	Mon, 19 Nov 2012 22:24:48 +0100
From:	Jan Kara <jack@...e.cz>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: The bug of iput() removal from flusher thread?

On Tue 20-11-12 05:51:15, OGAWA Hirofumi wrote:
> Jan Kara <jack@...e.cz> writes:
> 
> >>From 4fdc5d9a66dfe0286ef4f4a7f53fd3b15086470f Mon Sep 17 00:00:00 2001
> > From: Jan Kara <jack@...e.cz>
> > Date: Mon, 19 Nov 2012 20:01:16 +0100
> > Subject: [PATCH] writeback: Put unused inodes to LRU after writeback completion
> >
> > Commit 169ebd90 removed iget-iput pair from inode writeback. As a side effect,
> > inodes that are dirty during iput_final() call won't be ever added to inode LRU
> > (iput_final() doesn't add dirty inodes to LRU and later when the inode is
> > cleaned there's noone to add the inode there). Thus inodes are effectively
> > unreclaimable until someone looks them up again.
> >
> > Practical effect of this bug is limited by the fact that inodes are
> > pinned by a dentry for long enough that the inode gets cleaned. But still
> > the bug can have nasty consequences leading up to OOM conditions under
> > certain circumstances. Following can easily reproduce the problem:
> >
> > for (( i = 0; i < 1000; i++ )); do
> >   mkdir $i
> >   for (( j = 0; j < 1000; j++ )); do
> >     touch $i/$j
> >     echo 2 > /proc/sys/vm/drop_caches
> >   done
> > done
> >
> > then one needs to run 'sync; ls -lR' to make inodes reclaimable again.
> >
> > We fix the issue by inserting unused clean inodes into the LRU after writeback
> > finishes in inode_sync_complete().
> >
> > CC: Al Viro <viro@...iv.linux.org.uk>
> > Reported-by: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
> > Signed-off-by: Jan Kara <jack@...e.cz>
> 
> Need to Cc to stable@
  Yup, I'll add that. Thanks for reminder.

> > ---
> >  fs/fs-writeback.c |    3 +++
> >  fs/inode.c        |    2 +-
> >  fs/internal.h     |    1 +
> >  3 files changed, 5 insertions(+), 1 deletions(-)
> >
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index 51ea267..ed7613b 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -227,6 +227,9 @@ static void requeue_io(struct inode *inode, struct bdi_writeback *wb)
> >  
> >  static void inode_sync_complete(struct inode *inode)
> >  {
> > +	/* If inode is clean an unused, put it into LRU now.  */
> > +	if (!(inode->i_state & I_DIRTY) && !atomic_read(&inode->i_count))
> > +		inode_lru_list_add(inode);
> 
> IMHO, open coding this would be bad idea.
  Do you mean creating a separate function for the above two lines?

> And another one is I_REFERENCED. We really want to remove I_REFERENCED?
  We don't want I_REFERENCED set - noone used the inode. But looking into
the code with fresh eyes, the fix isn't as simple as I thought. First I
need to check MS_ACTIVE and second I need to check I_FREEING... So the
condition will be complex enough to warrant a separate function.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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