linux-ext4 - Re: [PATCH v2] ext4: fix race condition between buffer write and page

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Date:   Mon, 5 Jun 2023 17:08:55 +0200
From:   Jan Kara <jack@...e.cz>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jan Kara <jack@...e.cz>, Theodore Ts'o <tytso@....edu>,
        Baokun Li <libaokun1@...wei.com>, linux-ext4@...r.kernel.org,
        adilger.kernel@...ger.ca, ritesh.list@...il.com,
        linux-kernel@...r.kernel.org, jun.nie@...aro.org,
        ebiggers@...nel.org, yi.zhang@...wei.com, yangerkun@...wei.com,
        yukuai3@...wei.com,
        syzbot+a158d886ca08a3fecca4@...kaller.appspotmail.com,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] ext4: fix race condition between buffer write and
 page_mkwrite

On Mon 05-06-23 15:55:35, Matthew Wilcox wrote:
> On Mon, Jun 05, 2023 at 02:21:41PM +0200, Jan Kara wrote:
> > On Mon 05-06-23 11:16:55, Jan Kara wrote:
> > > Yeah, I agree, that is also the conclusion I have arrived at when thinking
> > > about this problem now. We should be able to just remove the conversion
> > > from ext4_page_mkwrite() and rely on write(2) or truncate(2) doing it when
> > > growing i_size.
> > 
> > OK, thinking more about this and searching through the history, I've
> > realized why the conversion is originally in ext4_page_mkwrite(). The
> > problem is described in commit 7b4cc9787fe35b ("ext4: evict inline data
> > when writing to memory map") but essentially it boils down to the fact that
> > ext4 writeback code does not expect dirty page for a file with inline data
> > because ext4_write_inline_data_end() should have copied the data into the
> > inode and cleared the folio's dirty flag.
> > 
> > Indeed messing with xattrs from the writeback path to copy page contents
> > into inline data xattr would be ... interesting. Hum, out of good ideas for
> > now :-|.
> 
> Is it so bad?  Now that we don't have writepage in ext4, only
> writepages, it seems like we have a considerably more benign locking
> environment to work in.

Well, yes, without ->writepage() it might be *possible*. But still rather
ugly. The problem is that in ->writepages() i_size is not stable. Thus also
whether the inode data is inline or not is not stable. We'd need inode_lock
for that but that is not easily doable in the writeback path - inode lock
would then become fs_reclaim unsafe...

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