Re: [PATCH v4 4/4] btrfs: cap shrink_delalloc iterations to 128M

[Boris Burkov <boris@bur.io>]

On Sat, Apr 25, 2026 at 07:51:01AM +0930, Qu Wenruo wrote:
> 
> 
> 在 2026/4/25 07:40, Boris Burkov 写道:
> > On Sat, Apr 25, 2026 at 07:36:49AM +0930, Qu Wenruo wrote:
> [...]
> > > At least we got something that both of us can reproduce.
> > > 
> > > Another thing is, for g/027 on arm64 I'm also actively monitoring the CPU
> > > usage through top.
> > > 
> > > Have you experienced very high (~100%) CPU usage on a kworker during g/027?
> > > 
> > 
> > No :(
> > As far as I can tell the system is stuck waiting on a commit. I'll keep
> > trying to repro your symptom.
> > 
> > I'm curious if it goes away for you with Sun's proposed fix, something
> > like setting pages to at least 1 after those to min() operations.
> 
> I go with the following diff:
> 
> diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
> index e931deb3d013..2c5214b24239 100644
> --- a/fs/btrfs/space-info.c
> +++ b/fs/btrfs/space-info.c
> @@ -770,6 +770,9 @@ static void shrink_delalloc(struct btrfs_space_info
> *space_info,
>                 u64 items = calc_reclaim_items_nr(fs_info, iter_reclaim) *
> 2;
>                 int async_pages;
> 
> +               if (nr_pages == 0)
> +                       nr_pages = 1;
> +
>                 btrfs_start_delalloc_roots(fs_info, nr_pages, true);
> 
>                 /*
> 
> 
> It solves the dead looping kworker on arm64, now the it's several different
> kworkers taking around 5~15% along with 027 itself taking CPU time.
> 
> But unfortunately the test case itself still seems not to end any time soon.
> 
> I believe the old 3 loops limit is really what makes the difference.
> It's ugly, but at least it seems to work.
> 
> Thanks,
> Qu

Thanks for re-testing. Glad to hear that at least fixes the dead loop.

Honestly I should have known better than to include an unbounded loop,
that was stupid. I even kind of thought it was dumb while doing it but
convinced myself it "must make progress one extent at a time" or
whatever.. Obviously overlooked the min() bug too.