Re: [PATCH] mm/page_alloc: restore 0-handling to zone_set_pageset_high_and_batch

[Joshua Hahn]

On Wed, 17 Dec 2025 12:23:58 +0100 Vlastimil Babka <vbabka@xxxxxxx> wrote:

> On 12/17/25 12:12, Vlastimil Babka wrote:
> > On 12/17/25 07:05, Joshua Hahn wrote:
> >> Commit 2783088ef24e ("mm/page_alloc: prevent reporting pcp->batch = 0")
> >> moved the error handling (0-handling) of zone_batchsize from its
> >> callers to inside the function. However, the commit left out the error
> >> handling for the NOMMU case, leading to deadlocks on NOMMU systems.
> >> 
> >> Since in the NOMMU case the reported-to-user batchsize should still be 0,
> > 
> > Should it? The value is effectively set to 1 despite what zone_batchsize()
> > returns, because of that adjustment this patch reinstates. Also does anyone
> > care, really?
> > 
> >> we would only like the error handling to exist in the callsites that
> >> set the internal value for the zone (i.e. zone_set_pageset_high_and_batch).
> >> 
> >> Restore max(1, zone_batchsize(zone)) to the callsite to prevent errors
> >> on NOMMU systems.
> > 
> > I would rather make zone_batchsize() for !CONFIG_MMU return 1 instead of 0.
> 
> Ah looks like you considered it too, initially:
> https://lore.kernel.org/all/20251211225947.822866-1-joshua.hahnjy@xxxxxxxxx/
> 
> It makes more sense to me than doing effectively two fixups in the MMU case.

Hi Vlastimil,

Thank you for your review as always.

Yes, I had also considered returning 1 for the !MMU case, since I think it
would make it a lot simpler as well (It would also make my original patch
function as intended).

However, I was unsure if changing this user-facing behavior for one line of
simplification would be worth it. I am not a NOMMU user, so I have very
little experience here, but I imagine that there is someone out there who
looks at zone_batchsize() returning 0 for NOMMU and interpreting it as
"there is no batching" as opposed to "there is batching, and it processes
1 page at a time" (which, actually isn't even true anyways because of the
bitshift). Maybe an option is to just make batchsize not visible
in the NOMMU case in addition to always returning 1 to avoid confusion.

Anyways, back to your original question of "does anyone care". . .

I am not sure : -)

For me, both solutions work, and in fact I prefer the original solution of
always reurning 1 for !NOMMU. Maybe some NOMMU users like Daniel and Guenter
can comment on whether this change really matters?

Thank you again for your review and follow-up. I hope you have a great day!
Joshua