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

[Oscar Salvador <osalvador@suse.de>]

On Mon, May 19, 2025 at 08:18:05PM +0300, 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.
> 
> 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@amazon.de>
> Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Reviewed-by: Oscar Salvador <osalvador@suse.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.


-- 
Oscar Salvador
SUSE Labs