Re: [PATCH v2] mm/page_alloc: Report 1 as zone_batchsize for !CONFIG_MMU

[Hajime Tazaki <thehajime@gmail.com>]

Hello Daniel,

On Thu, 18 Dec 2025 06:30:42 -0600,
Daniel Palmer wrote:
> 
> Hi Joshua,
> 
> On Thu, 18 Dec 2025 at 17:32, Joshua Hahn <joshua.hahnjy@gmail.com> 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.
> >
> > For NOMMU systems, return 1 instead of 0 for zone_batchsize, which restores
> > the previous deadlock-free behavior.
> 
> Tested this on my 68000 setup, filled the memory to cause an OOM and I
> got OOM instead of deadlock as expected.
> 
> Tested-by: Daniel Palmer <daniel@thingy.jp>
> 
> FWIW There was a BoF about NOMMU at LPC last week and I did mention to
> the people presenting that seem to be using NOMMU in real world
> applications that NOMMU was broken in mainline. I hoped they would
> have chimed in on this..

I tested with UML with nommu extension (currently out of kernel *1)
and reproduced the issue with a crafted program causing OOM.

without patch it indeed hangs up with losing console access and this
patch fixes with a proper failure message like below;

oom: page allocation failure: order:12, mode:0xcc0(GFP_KERNEL), nodemask=(null)
CPU: 0 UID: 0 PID: 32 Comm: oom Not tainted 6.18.0-12966-gc43a4f128407-dirty #223 NONE
Stack:
 60a8fb80 604a246e 603b9569 00000001
 ffffff00 604a246e 6002440d 604a1479
 60a8fbb0 6002bbb3 60556910 00000000
Call Trace:
 [<6002440d>] ? _printk+0x0/0x5b
 [<6002df89>] show_stack+0x11c/0x12b
 [<603b9569>] ? dump_stack_print_info+0x0/0x12f
 [<6002440d>] ? _printk+0x0/0x5b
 [<6002bbb3>] dump_stack_lvl+0x65/0x80
 [<6002bbec>] dump_stack+0x1e/0x20
 [<600e0c13>] warn_alloc+0x118/0x195
 [<60083ae0>] ? __mutex_trylock+0x16/0x1e
(snip)


Tested-by: Hajime Tazaki <thehajime@gmail.com>

*1 https://lore.kernel.org/all/cover.1762588860.git.thehajime@gmail.com/

-- Hajime