linux-ext4 - Re: [RFC PATCH] ext4: dio take shared inode lock when overwriting preallocated blocks

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Message-ID: <20221215084850.abze2sz2imwcoma5@quack3>
Date:   Thu, 15 Dec 2022 09:48:50 +0100
From:   Jan Kara <jack@...e.cz>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Jan Kara <jack@...e.cz>, Zhang Yi <yi.zhang@...wei.com>,
        linux-ext4@...r.kernel.org, adilger.kernel@...ger.ca,
        yi.zhang@...weicloud.com, yukuai3@...wei.com
Subject: Re: [RFC PATCH] ext4: dio take shared inode lock when overwriting
 preallocated blocks

On Wed 14-12-22 13:52:32, Theodore Ts'o wrote:
> On Wed, Dec 14, 2022 at 06:01:25PM +0100, Jan Kara wrote:
> > 
> > Besides some naming nits (see below) I think this should work. But I have
> > to say I'm a bit uneasy about this because we will now be changing block
> > mapping from unwritten to written only with shared i_rwsem. OTOH that
> > happens during writeback as well so we should be fine and the gain is very
> > nice.
> 
> Hmm.... when I was looking potential impacts of the change what
> ext4_overwrite_io() would do, I looked at the current user of that
> function in ext4_dio_write_checks().
> 
> 	/*
> 	 * Determine whether the IO operation will overwrite allocated
> 	 * and initialized blocks.
> 	 * We need exclusive i_rwsem for changing security info
> 	 * in file_modified().
> 	 */
> 	if (*ilock_shared && (!IS_NOSEC(inode) || *extend ||
> 	     !ext4_overwrite_io(inode, offset, count))) {
> 		if (iocb->ki_flags & IOCB_NOWAIT) {
> 			ret = -EAGAIN;
> 			goto out;
> 		}
> 		inode_unlock_shared(inode);
> 		*ilock_shared = false;
> 		inode_lock(inode);
> 		goto restart;
> 	}
> 
> 	ret = file_modified(file);
> 	if (ret < 0)
> 		goto out;
> 
> What is confusing me is the comment, "We need exclusive i_rwsem for
> changing security info in file_modified().".  But then we end up
> calling file_modified() unconditionally, regardless of whether we've
> transitioned from a shared lock to an exclusive lock.
> 
> So file_modified() can get called either with or without the inode
> locked r/w.  I realize that this patch doesn't change this
> inconsistency, but it appears either the comment is wrong, or the code
> is wrong.

Maybe the comment needs rephrasing but it seems correct. file_modified()
does multiple things. It updates timestamps - these are fine with shared
i_rwsem - and is calls into __file_remove_privs() to remove SUID bits etc.
Now if __file_remove_privs() is going to modify the inode, we need i_rwsem
exclusively. And we determine whether __file_remove_privs() will do
anything by checking !IS_NOSEC(inode) in the condition above. So the
sentence you're confused about speaks about this part of the condition.

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