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Date:   Mon, 6 Jun 2022 10:38:14 +0200
From:   Jan Kara <jack@suse.cz>
To:     "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc:     Jan Kara <jack@suse.com>, tytso@mit.edu,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 3/3] ext4: Use generic_quota_read()
Message-ID: <20220606083814.skjv34b2tjn7l7pi@quack3.lan>
References: <20220605143815.2330891-1-willy@infradead.org>
 <20220605143815.2330891-4-willy@infradead.org>
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In-Reply-To: <20220605143815.2330891-4-willy@infradead.org>
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On Sun 05-06-22 15:38:15, Matthew Wilcox (Oracle) wrote:
> The comment about the page cache is rather stale; the buffer cache will
> read into the page cache if the buffer isn't present, and the page cache
> will not take any locks if the page is present.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

This will not work for couple of reasons, see below. BTW, I don't think the
comment about page cache was stale (but lacking details I admit ;). As far
as I remember (and it was really many years ago - definitely pre-git era)
the problem was (mainly on the write side) that before current state of the
code we were using calls like vfs_read() / vfs_write() to get quota
information and that was indeed prone to deadlocks.

> @@ -6924,20 +6882,21 @@ static ssize_t ext4_quota_write(struct super_block *sb, int type,
>  		return -EIO;
>  	}
>  
> -	do {
> -		bh = ext4_bread(handle, inode, blk,
> -				EXT4_GET_BLOCKS_CREATE |
> -				EXT4_GET_BLOCKS_METADATA_NOFAIL);
> -	} while (PTR_ERR(bh) == -ENOSPC &&
> -		 ext4_should_retry_alloc(inode->i_sb, &retries));
> -	if (IS_ERR(bh))
> -		return PTR_ERR(bh);
> -	if (!bh)
> +	folio = read_mapping_folio(inode->i_mapping, off / PAGE_SIZE, NULL);
> +	if (IS_ERR(folio))
> +		return PTR_ERR(folio);
> +	head = folio_buffers(folio);
> +	if (!head)
> +		head = alloc_page_buffers(&folio->page, sb->s_blocksize, false);
> +	if (!head)
>  		goto out;
> +	bh = head;
> +	while ((bh_offset(bh) + sb->s_blocksize) <= (off % PAGE_SIZE))
> +		bh = bh->b_this_page;

We miss proper handling of blocks that are currently beyond i_size
(we are extending the quota file), plus we also miss any mapping of buffers
to appropriate disk blocks here...

It could be all fixed by replicating what we do in ext4_write_begin() but
I'm not quite convinced using inode's page cache is really worth it...

								Honza

>  	BUFFER_TRACE(bh, "get write access");
>  	err = ext4_journal_get_write_access(handle, sb, bh, EXT4_JTR_NONE);
>  	if (err) {
> -		brelse(bh);
> +		folio_put(folio);
>  		return err;
>  	}
>  	lock_buffer(bh);
> @@ -6945,14 +6904,14 @@ static ssize_t ext4_quota_write(struct super_block *sb, int type,
>  	flush_dcache_page(bh->b_page);
>  	unlock_buffer(bh);
>  	err = ext4_handle_dirty_metadata(handle, NULL, bh);
> -	brelse(bh);
>  out:
> +	folio_put(folio);
> +	if (err)
> +		return err;
>  	if (inode->i_size < off + len) {
>  		i_size_write(inode, off + len);
>  		EXT4_I(inode)->i_disksize = inode->i_size;
> -		err2 = ext4_mark_inode_dirty(handle, inode);
> -		if (unlikely(err2 && !err))
> -			err = err2;
> +		err = ext4_mark_inode_dirty(handle, inode);
>  	}
>  	return err ? err : len;
>  }
> -- 
> 2.35.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR