linux-kernel - Re: [PATCH] mm/cma: make detection of highmem

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Message-ID: <d2751191-fc32-418a-8b62-dedab41d0615@redhat.com>
Date: Tue, 20 May 2025 11:14:28 +0200
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...e.de>, Mike Rapoport <rppt@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Alexandre Ghiti <alexghiti@...osinc.com>, Pratyush Yadav
 <ptyadav@...zon.de>, linux-arch@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/cma: make detection of highmem_start more robust

On 20.05.25 10:30, Oscar Salvador wrote:
> On Mon, May 19, 2025 at 08:18:05PM +0300, Mike Rapoport wrote:
>> From: "Mike Rapoport (Microsoft)" <rppt@...nel.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.
>>
>> And in the case CONFIG_HIGHMEM is disabled high_memory essentially becomes
>> the first address after memory end, so instead of relying on high_memory to
>> calculate highmem_start use memblock_end_of_DRAM() and eliminate the
>> dependency of CMA area creation on high_memory in majority of
>> configurations.
>>
>> Reported-by: Pratyush Yadav <ptyadav@...zon.de>
>> Tested-by: Alexandre Ghiti <alexghiti@...osinc.com>
>> Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> 
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> 
> I will note though that it is a bit akward to have highmem involved here
> when we might not have CONFIG_HIGHMEM enabled.
> I get that for !CONFIG_HIGHMEM it is a no-op situation, but still I
> wonder whether we could abstract that from this function.

Same thought here.

Can't we do some IS_ENABLED(CONFIG_HIGHMEM) magic or similar to not even 
use that variable without CONFIG_HIGHMEM?

-- 
Cheers,

David / dhildenb