Re: [PATCH v2 4/4] mm: page_alloc: fix non-movable reclaim storm in defrag_mode

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Jul 23, 2026 at 12:22:58PM +0200, Vlastimil Babka (SUSE) wrote:
> + Greg and Sasha
> 
> On 7/23/26 01:22, Andrew Morton wrote:
> > On Wed, 22 Jul 2026 10:56:47 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:
> > 
> >> 
> >> Fixes: e3aa7df331bc ("mm: page_alloc: defrag_mode")
> >> Cc: <stable@vger.kernel.org>
> >> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > 
> > This cc:stable fix doesn't apply to current mainline because the
> > preceding non-cc:stable patches get in the way.
> > 
> > This means that the -stable maintainers will have to ask you to redo
> > the fix to make it backportable.  And you'll then be preparing a patch
> > which hasn't had standalone testing in mainline.  All very cumbersome.
> 
> Or they backport the whole series, even if only 4/4 has a Fixes: tag.
> 
> > So is it practical to make the fix come first, as a cc:stable hotfix so
> > we can upstream the other patches in the next merge window?
> 
> I think it can't come first, it's simply a complex fix that can't be done in
> a single patch that would be reviewable. 3/4 is certainly a strict
> dependency. I'm not sure if 2/4 as well. Perhaps 2/4 does deserve a Fixes:
> tag and Cc: stable on its own, as it also fixes a defrag_mode deficiency.

Yes. Only 1/4 could be argued away.

2/4 actually came after 4/4 during development. Without it, compaction
cannot make sufficient forward progress on the promoted non-movable
requests. So it is a hard functional requirement.

Whether it deserves an independent Fixes: depends on how many
GFP_KERNEL pageblock_order requests we have :-) I'm guessing not many.

> > (An alternative would be to put cc:stable on all four patches.  That's
> > a bit abusive but we do it rarely).
> 
> I don't think it's necessary, the stable team are quite capable of figuring
> out the deps and backporting them with a Stable-dep-off: tag. IIRC the
> "fake" tagging is discouraged? Maybe they can clarify.

Yes, I *think* it should work with tagging just 4/4.

It won't apply cleanly or build without 3/4. So I'd assume if that
dependency is lost, the automated backport will bounce and I can send
a backport series by hand.