linux-kernel - Re: [PATCH v2 5/6] ext4/jbd2: reintroduce jbd2_journal_blocks_per

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Message-ID: <6koueljloyoehqg4my5ihbvybid4unimyiahqcbkoasx5cqtat@xehauy7m2ofq>
Date: Mon, 23 Jun 2025 09:26:15 +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, yi.zhang@...wei.com, libaokun1@...wei.com, 
	yukuai3@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH v2 5/6] ext4/jbd2: reintroduce
 jbd2_journal_blocks_per_page()

On Sat 21-06-25 15:46:40, Zhang Yi wrote:
> On 2025/6/20 0:44, Jan Kara wrote:
> > On Wed 11-06-25 19:16:24, Zhang Yi wrote:
> >> From: Zhang Yi <yi.zhang@...wei.com>
> >>
> >> This partially reverts commit d6bf294773a4 ("ext4/jbd2: convert
> >> jbd2_journal_blocks_per_page() to support large folio"). This
> >> jbd2_journal_blocks_per_folio() will lead to a significant
> >> overestimation of journal credits. Since we still reserve credits for
> >> one page and attempt to extend and restart handles during large folio
> >> writebacks, so we should convert this helper back to
> >> ext4_journal_blocks_per_page().
> >>
> >> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> > 
> > Here I'm not decided. Does it make any particular sense to reserve credits
> > for one *page* worth of blocks when pages don't have any particular meaning
> > in our writeback code anymore? We could reserve credits just for one
> > physical extent and that should be enough.
> 
> Indeed, reserving credits for a single page is no longer suitable in the
> currently folio based context. It do seems more appropriate to allocate
> credits for a single extent.
> 
> > For blocksize == pagesize (most
> > common configs) this would be actually equivalent. If blocksize < pagesize,
> > this could force us to do some more writeback retries and thus get somewhat
> > higher writeback CPU overhead but do we really care for these configs?  It
> > is well possible I've overlooked something and someone will spot a
> > performance regression in practical setup with this in which case we'd have
> > to come up with something more clever but I think it's worth it to start
> > simple and complicate later.
> 
> This can indeed be a problem if the file system is already fragmented
> enough. However, thanks to the credits extension logic in
> __ext4_journal_ensure_credits(), I suppose that on most file system images,
> it will not trigger excessive retry operations. Besides, although there
> might be some lock cost in jbd2_journal_extend(), I suppose it won't be a
> big deal.
> 
> Perhaps we could reserve more credits through a complex formula at the
> outset, which would lower the cost of expanding the credits. But I don't
> think this will help much in reducing the number of retries, it may only
> be helpful in extreme cases (the running transaction stats to commit, we
> cannot extend it).
> 
> So, I think we can implement it by reserving for an extent for the time
> being. Do you agree?

Yes, I agree. After all this is easy to change if we find some real issues
with it.

								Honza

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