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

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Message-ID: <mk5f4g4rwp37ob6qmd7asocumeepjcnufqzjvazr3yukbyzq3y@6gp6x6zvxf7r>
Date: Fri, 4 Jul 2025 10:18:56 +0200
From: Jan Kara <jack@...e.cz>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, tytso@....edu, 
	adilger.kernel@...ger.ca, ojaswin@...ux.ibm.com, sashal@...nel.org, yi.zhang@...wei.com, 
	libaokun1@...wei.com, yukuai3@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH v3 05/10] ext4: restart handle if credits are
 insufficient during allocating blocks

On Fri 04-07-25 09:40:23, Zhang Yi wrote:
> On 2025/7/4 0:27, Jan Kara wrote:
> > 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@...wei.com>
> >>>>
> >>>> 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@...e.cz>
> >>>> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> >>>
> >>> Very nice. Feel free to add:
> >>>
> >>> Reviewed-by: Jan Kara <jack@...e.cz>
> >>>
> >>> 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.
> > 
> 
> Yeah, remove this test is fine with me. I added this one is because the
> comments in ext4_handle_valid() said "Do not use this for NULL handles."
> I think it is best to follow this rule. :)

Right, I didn't read that comment :) So maybe the best fix will be just
adding a comment before the test like:

	/* Called from ext4_da_write_begin() which has no handle started? */
	if (!handle)
		return 0;

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