linux-ext4 - Re: [PATCH] ext4: Fix warning in ext4_dio_write_end

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Message-ID: <20231123084954.oegpgspqm37nnz2a@quack3>
Date: Thu, 23 Nov 2023 09:49:54 +0100
From: Jan Kara <jack@...e.cz>
To: Ritesh Harjani <ritesh.list@...il.com>
Cc: Jan Kara <jack@...e.cz>, Ted Tso <tytso@....edu>,
	linux-ext4@...r.kernel.org,
	syzbot+47479b71cdfc78f56d30@...kaller.appspotmail.com
Subject: Re: [PATCH] ext4: Fix warning in ext4_dio_write_end_io()

On Thu 23-11-23 12:37:03, Ritesh Harjani wrote:
> Jan Kara <jack@...e.cz> writes:
> 
> > The syzbot has reported that it can hit the warning in
> > ext4_dio_write_end_io() because i_size < i_disksize. Indeed the
> > reproducer creates a race between DIO IO completion and truncate
> > expanding the file and thus ext4_dio_write_end_io() sees an inconsistent
> > inode state where i_disksize is already updated but i_size is not
> > updated yet. Since we are careful when setting up DIO write and consider
> > it extending (and thus performing the IO synchronously with i_rwsem held
> > exclusively) whenever it goes past either of i_size or i_disksize, we
> > can use the same test during IO completion without risking entering
> > ext4_handle_inode_extension() without i_rwsem held. This way we make it
> > obvious both i_size and i_disksize are large enough when we report DIO
> > completion without relying on unreliable WARN_ON.
> 
> Does it make sense to add this in ext4_handle_inode_extension()?
> 	WARN_ON_ONCE(!inode_is_locked(inode));
> Ohk, we already have "lockdep_assert_held_write(&inode->i_rwsem)" so
> hopefully it can catch via lockdep.

Exactly.
 
> So, IIUC, the WARN happened when we were doing a non-extending
> AIO-DIO write which was racing with truncate trying to expand the file
> size. Because only then the DIO completion will not have i_rwsem held
> which can race with truncate. Truncate since it is expanding the file
> size, will not use inode_dio_wait() (since no block allocations).
> 
> Is this understanding correct?

Yes, correct.

								Honza

> 
> >
> > Reported-by: syzbot+47479b71cdfc78f56d30@...kaller.appspotmail.com
> > Fixes: 91562895f803 ("ext4: properly sync file size update after O_SYNC direct IO")
> > Signed-off-by: Jan Kara <jack@...e.cz>
> > ---
> >  fs/ext4/file.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> > index 0166bb9ca160..ba497aabdd1e 100644
> > --- a/fs/ext4/file.c
> > +++ b/fs/ext4/file.c
> > @@ -386,10 +386,11 @@ static int ext4_dio_write_end_io(struct kiocb *iocb, ssize_t size,
> >  	 * blocks. But the code in ext4_iomap_alloc() is careful to use
> >  	 * zeroed/unwritten extents if this is possible; thus we won't leave
> >  	 * uninitialized blocks in a file even if we didn't succeed in writing
> > -	 * as much as we intended.
> > +	 * as much as we intended. Also we can race with truncate or write
> > +	 * expanding the file so we have to be a bit careful here.
> >  	 */
> > -	WARN_ON_ONCE(i_size_read(inode) < READ_ONCE(EXT4_I(inode)->i_disksize));
> > -	if (pos + size <= READ_ONCE(EXT4_I(inode)->i_disksize))
> > +	if (pos + size <= READ_ONCE(EXT4_I(inode)->i_disksize) &&
> > +	    pos + size <= i_size_read(inode))
> >  		return size;
> >  	return ext4_handle_inode_extension(inode, pos, size);
> >  }
> > -- 
> > 2.35.3
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR