On Wed 11-06-25 19:16:22, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
>
> After large folios are supported on ext4, writing back a sufficiently
> large and discontinuous folio may consume a significant number of
> journal credits, placing considerable strain on the journal. For
> example, in a 20GB filesystem with 1K block size and 1MB journal size,
> writing back a 2MB folio could require thousands of credits in the
> worst-case scenario (when each block is discontinuous and distributed
> across different block groups), potentially exceeding the journal size.
> This issue can also occur in ext4_write_begin() and ext4_page_mkwrite()
> when delalloc is not enabled.
>
> Fix this by ensuring that there are sufficient journal credits before
> allocating an extent in mpage_map_one_extent() and _ext4_get_block(). If
> there are not enough credits, return -EAGAIN, exit the current mapping
> loop, restart a new handle and a new transaction, and allocating blocks
> on this folio again in the next iteration.
>
> Suggested-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
...
> static int _ext4_get_block(struct inode *inode, sector_t iblock,
> struct buffer_head *bh, int flags)
> {
> struct ext4_map_blocks map;
> + handle_t *handle = ext4_journal_current_handle();
> int ret = 0;
>
> if (ext4_has_inline_data(inode))
> return -ERANGE;
>
> + /* Make sure transaction has enough credits for this extent */
> + if (flags & EXT4_GET_BLOCKS_CREATE) {
> + ret = ext4_journal_ensure_extent_credits(handle, inode);
> + if (ret)
> + return ret;
> + }
> +
> map.m_lblk = iblock;
> map.m_len = bh->b_size >> inode->i_blkbits;
>
> - ret = ext4_map_blocks(ext4_journal_current_handle(), inode, &map,
> - flags);
> + ret = ext4_map_blocks(handle, inode, &map, flags);
Good spotting with ext4_page_mkwrite() and ext4_write_begin() also needing
this treatment! But rather then hiding the transaction extension in
_ext4_get_block() I'd do this in ext4_block_write_begin() where it is much
more obvious (and also it is much more obvious who needs to be prepared for
handling EAGAIN error). Otherwise the patch looks good!
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
