linux-ext4 - Re: [PATCH] ext4: fix reservation overflow in ext4_da_write

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Date:	Thu, 2 Oct 2014 15:00:23 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: fix reservation overflow in ext4_da_write_begin

On Oct 2, 2014, at 9:28 AM, Eric Sandeen <sandeen@...hat.com> wrote:
> Delalloc write journal reservations only reserve 1 credit,
> to update the inode if necessary.  However, it may happen
> once in a filesystem's lifetime that a file will cross
> the 2G threshold, and require the LARGE_FILE feature to
> be set in the superblock as well, if it was not set already.
> 
> This overruns the transaction reservation, and can be
> demonstrated simply on any ext4 filesystem without the LARGE_FILE
> feature already set:
> 
> dd if=/dev/zero of=testfile bs=1 seek=2147483646 count=1 \
> 	conv=notrunc of=testfile
> sync
> dd if=/dev/zero of=testfile bs=1 seek=2147483647 count=1 \
> 	conv=notrunc of=testfile
> 
> leads to:
> 
> EXT4-fs: ext4_do_update_inode:4296: aborting transaction: error 28 in __ext4_handle_dirty_super
> EXT4-fs error (device loop0) in ext4_do_update_inode:4301: error 28
> EXT4-fs error (device loop0) in ext4_reserve_inode_write:4757: Readonly filesystem
> EXT4-fs error (device loop0) in ext4_dirty_inode:4876: error 28
> EXT4-fs error (device loop0) in ext4_da_write_end:2685: error 28
> 
> Adjust the number of credits based on whether the flag is
> already set, and whether the current write may extend past the
> LARGE_FILE limit.
> 
> Signed-off-by: Eric Sandeen <sandeen@...hat.com>

Reviewed-by: Andreas Dilger <adilger@...ger.ca>

> --- 
> 
> Ok, how's this ... I do like this a lot better than the set-flag-on-
> mount-or-remount, which started to get a bit icky.
> 
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 3aa26e9..8d362c2 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -2515,6 +2515,20 @@ static int ext4_nonda_switch(struct super_block *sb)
> 	return 0;
> }
> 
> +/* We always reserve for an inode update; the superblock could be there too */
> +static int ext4_da_write_credits(struct inode *inode, loff_t pos, unsigned len)
> +{
> +	if (EXT4_HAS_RO_COMPAT_FEATURE(inode->i_sb,

This could be marked "likely()" I suspect, but not critical.

> +                                EXT4_FEATURE_RO_COMPAT_LARGE_FILE))
> +		return 1;
> +
> +	if (pos + len <= 0x7fffffffULL)
> +		return 1;
> +
> +	/* We might need to update the superblock to set LARGE_FILE */
> +	return 2;
> +}
> +
> static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
> 			       loff_t pos, unsigned len, unsigned flags,
> 			       struct page **pagep, void **fsdata)
> @@ -2565,7 +2579,8 @@ retry_grab:
> 	 * of file which has an already mapped buffer.
> 	 */
> retry_journal:
> -	handle = ext4_journal_start(inode, EXT4_HT_WRITE_PAGE, 1);
> +	handle = ext4_journal_start(inode, EXT4_HT_WRITE_PAGE,
> +			ext4_da_write_credits(inode, pos, len));
> 	if (IS_ERR(handle)) {
> 		page_cache_release(page);
> 		return PTR_ERR(handle);
> 
> --
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Cheers, Andreas






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