Re: [PATCH v3 05/10] ext4: restart handle if credits are insufficient during allocating blocks

[Jan Kara]

On Thu 03-07-25 10:13:07, Zhang Yi wrote:
> On 2025/7/2 22:18, Jan Kara wrote:
> > On Tue 01-07-25 21:06:30, 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_block_write_begin(). 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>
> > 
> > Very nice. Feel free to add:
> > 
> > Reviewed-by: Jan Kara <jack@xxxxxxx>
> > 
> > One small comment below:
> > 
> >> +/*
> >> + * Make sure that the current journal transaction has enough credits to map
> >> + * one extent. Return -EAGAIN if it cannot extend the current running
> >> + * transaction.
> >> + */
> >> +static inline int ext4_journal_ensure_extent_credits(handle_t *handle,
> >> +						     struct inode *inode)
> >> +{
> >> +	int credits;
> >> +	int ret;
> >> +
> >> +	if (!handle)
> > 
> > Shouldn't this rather be ext4_handle_valid(handle) to catch nojournal mode
> > properly?
> > 
> __ext4_journal_ensure_credits() already calls ext4_handle_valid() to handle
> nojournal mode, and the '!handle' check here is to handle the case where
> ext4_block_write_begin() passes in a NULL 'handle'.

Ah, right. But then you don't need the test at all, do you? Anyway,
whatever you decide to do with this (or nothing) is fine by me.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR