Re: [PATCH] mm/cma: make detection of highmem_start more robust

[Mike Rapoport <rppt@kernel.org>]

On Mon, May 19, 2025 at 11:55:05PM +0200, Pratyush Yadav wrote:
> Hi Mike,
> 
> On Mon, May 19 2025, Mike Rapoport wrote:
> 
> > From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
> >
> > Pratyush Yadav reports the following crash:
> >
> >     ------------[ cut here ]------------
> >     kernel BUG at arch/x86/mm/physaddr.c:23!
> >     ception 0x06 IP 10:ffffffff812ebbf8 error 0 cr2 0xffff88903ffff000
> >     CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 6.15.0-rc6+ #231 PREEMPT(undef)
> >     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014
> >     RIP: 0010:__phys_addr+0x58/0x60
> >     Code: 01 48 89 c2 48 d3 ea 48 85 d2 75 05 e9 91 52 cf 00 0f 0b 48 3d ff ff ff 1f 77 0f 48 8b 05 20 54 55 01 48 01 d0 e9 78 52 cf 00 <0f> 0b 90 0f 1f 44 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90
> >     RSP: 0000:ffffffff82803dd8 EFLAGS: 00010006 ORIG_RAX: 0000000000000000
> >     RAX: 000000007fffffff RBX: 00000000ffffffff RCX: 0000000000000000
> >     RDX: 000000007fffffff RSI: 0000000280000000 RDI: ffffffffffffffff
> >     RBP: ffffffff82803e68 R08: 0000000000000000 R09: 0000000000000000
> >     R10: ffffffff83153180 R11: ffffffff82803e48 R12: ffffffff83c9aed0
> >     R13: 0000000000000000 R14: 0000001040000000 R15: 0000000000000000
> >     FS:  0000000000000000(0000) GS:0000000000000000(0000) knlGS:0000000000000000
> >     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >     CR2: ffff88903ffff000 CR3: 0000000002838000 CR4: 00000000000000b0
> >     Call Trace:
> >      <TASK>
> >      ? __cma_declare_contiguous_nid+0x6e/0x340
> >      ? cma_declare_contiguous_nid+0x33/0x70
> >      ? dma_contiguous_reserve_area+0x2f/0x70
> >      ? setup_arch+0x6f1/0x870
> >      ? start_kernel+0x52/0x4b0
> >      ? x86_64_start_reservations+0x29/0x30
> >      ? x86_64_start_kernel+0x7c/0x80
> >      ? common_startup_64+0x13e/0x141
> >
> >   The reason is that __cma_declare_contiguous_nid() does:
> >
> >           highmem_start = __pa(high_memory - 1) + 1;
> >
> >   If dma_contiguous_reserve_area() (or any other CMA declaration) is
> >   called before free_area_init(), high_memory is uninitialized. Without
> >   CONFIG_DEBUG_VIRTUAL, it will likely work but use the wrong value for
> >   highmem_start.
> >
> > The issue occurs because commit e120d1bc12da ("arch, mm: set high_memory in
> > free_area_init()") moved initialization of high_memory after the call to
> > dma_contiguous_reserve() -> __cma_declare_contiguous_nid() on several
> > architectures.
> >
> > In the case CONFIG_HIGHMEM is enabled, some architectures that actually
> > support HIGHMEM (arm, powerpc and x86) have initialization of high_memory
> > before a possible call to __cma_declare_contiguous_nid() and some
> > initialized high_memory late anyway (arc, csky, microblase, mips, sparc,
> > xtensa) even before the commit e120d1bc12da so they are fine with using
> > uninitialized value of high_memory.
> 
> I don't know if they are fine or they haven't realized this is a bug
> yet.

For those that initialized high_memory in their mem_init() it would have
been a bug quite some time.

> Either way, this patch fixes the crash for me on x86_64, restores how it
> should behave, and doesn't seem to make anything worse, so:
> 
> Tested-by: Pratyush Yadav <ptyadav@amazon.de>

Thanks!
 
> Thanks for fixing this!
> 
> -- 
> Regards,
> Pratyush Yadav

-- 
Sincerely yours,
Mike.