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

[Pratyush Yadav <ptyadav@amazon.de>]

Hi Mike,

On Tue, May 20 2025, Mike Rapoport wrote:

> 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.

Agreed. This patch fixes the regression caused by e120d1bc12da5 ("arch,
mm: set high_memory in free_area_init()"). I don't think you need to go
around fixing long standing bugs. I was just thinking out loud with this
comment.

[...]

-- 
Regards,
Pratyush Yadav



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