linux-kernel - Re: [PATCH v2 10/13] arch, mm: set high_memory in free_area

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Message-ID: <20250402181842-f25872a1-00f7-4a8f-ae6d-3927899ee3a6@linutronix.de>
Date: Wed, 2 Apr 2025 18:31:02 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Mike Rapoport <rppt@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, 
	Ard Biesheuvel <ardb@...nel.org>, Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>, 
	"David S. Miller" <davem@...emloft.net>, Geert Uytterhoeven <geert@...ux-m68k.org>, 
	Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org, 
	linux-mm@...ck.org, x86@...nel.org, Naresh Kamboju <naresh.kamboju@...aro.org>, 
	lkft-triage@...ts.linaro.org, Linux Regressions <regressions@...ts.linux.dev>
Subject: Re: [PATCH v2 10/13] arch, mm: set high_memory in free_area_init()

On Wed, Apr 02, 2025 at 03:07:51PM +0200, Thomas Weißschuh wrote:
> On Wed, Apr 02, 2025 at 03:46:37PM +0300, Mike Rapoport wrote:
> > On Wed, Apr 02, 2025 at 02:19:01PM +0200, Thomas Weißschuh wrote:
> > > (drop all the non-x86 and non-mm recipients)
> > > 
> > > On Thu, Mar 13, 2025 at 03:50:00PM +0200, Mike Rapoport wrote:
> > > > From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
> > > > 
> > > > high_memory defines upper bound on the directly mapped memory.
> > > > This bound is defined by the beginning of ZONE_HIGHMEM when a system has
> > > > high memory and by the end of memory otherwise.
> > > > 
> > > > All this is known to generic memory management initialization code that
> > > > can set high_memory while initializing core mm structures.
> > > > 
> > > > Add a generic calculation of high_memory to free_area_init() and remove
> > > > per-architecture calculation except for the architectures that set and
> > > > use high_memory earlier than that.
> > > 
> > > This change (in mainline as commit e120d1bc12da ("arch, mm: set high_memory in free_area_init()")
> > > breaks booting i386 on QEMU for me (and others [0]).
> > > The boot just hangs without output.
> > > 
> > > It's easily reproducible with kunit:
> > > ./tools/testing/kunit/kunit.py run --arch i386
> > > 
> > > See below for the specific problematic hunk.
> > > 
> > > [0] https://lore.kernel.org/lkml/CA+G9fYtdXHVuirs3v6at3UoKNH5keuq0tpcvpz0tJFT4toLG4g@mail.gmail.com/
> > > 
> > > 
> > > > diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
> > > > index 6d2f8cb9451e..801b659ead0c 100644
> > > > --- a/arch/x86/mm/init_32.c
> > > > +++ b/arch/x86/mm/init_32.c
> > > > @@ -643,9 +643,6 @@ void __init initmem_init(void)
> > > >  		highstart_pfn = max_low_pfn;
> > > >  	printk(KERN_NOTICE "%ldMB HIGHMEM available.\n",
> > > >  		pages_to_mb(highend_pfn - highstart_pfn));
> > > > -	high_memory = (void *) __va(highstart_pfn * PAGE_SIZE - 1) + 1;
> > > > -#else
> > > > -	high_memory = (void *) __va(max_low_pfn * PAGE_SIZE - 1) + 1;
> > > >  #endif
> > > 
> > > Reverting this hunk fixes the issue for me.
> >  
> > This is already done by d893aca973c3 ("x86/mm: restore early initialization
> > of high_memory for 32-bits").
> 
> Thanks. Of course I only noticed this shortly after sending my mail.
> But this usecase is indeed broken on mainline.
> Some further bisecting lead to the mm merge commit being broken, while both its
> parents work. That lead the bisection astray.
> eb0ece16027f ("Merge tag 'mm-stable-2025-03-30-16-52' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm")
> 
> As unlikely as it sounds, it's reproducible. I'll investigate a bit.

The issue is fixed with the following diff:

diff --git a/mm/memblock.c b/mm/memblock.c
index 284154445409..8cd95f60015d 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -2165,7 +2165,8 @@ static unsigned long __init __free_memory_core(phys_addr_t start,
                                 phys_addr_t end)
 {
        unsigned long start_pfn = PFN_UP(start);
-       unsigned long end_pfn = PFN_DOWN(end);
+       unsigned long end_pfn = min_t(unsigned long,
+                                     PFN_DOWN(end), max_low_pfn);

        if (start_pfn >= end_pfn)
                return 0;


Background:

This reverts part of commit 6faea3422e3b ("arch, mm: streamline HIGHMEM freeing")
which is the direct child of the partially reverted 
commit e120d1bc12da ("arch, mm: set high_memory in free_area_init()").
The assumptions the former commit became invalid with the partial revert the latter.

This bug only triggers when CONFIG_HIGHMEM=n. When mm was branched from mainline
the i386 configuration generated by kunit ended up with CONFIG_HIGHMEM=y.
With some recent changes in mainline the kunit configuration switched to
CONFIG_HIGHMEM=n, triggering this specific reproducer only when mm got merged
into mainline again.

New kunit reproducer:
./tools/testing/kunit/kunit.py run --arch i386 example --timeout 10 --kconfig_add CONFIG_HIGHMEM=n

Does this sound reasonable?  If so I'll send a patch tomorrow.

@Naresh, could you test this, too?


Thomas