Re: [PATCH] ALSA: ctxfi: Limit PTP to a single page

[Takashi Iwai]

On Mon, 06 Apr 2026 09:48:57 +0200,
Harin Lee wrote:
> 
> Commit 391e69143d0a increased CT_PTP_NUM from 1 to 4 to support 256
> playback streams, but the additional pages are not used by the card
> correctly. The CT20K2 hardware already has multiple VMEM_PTPAL
> registers, but using them separately would require refactoring the
> entire virtual memory allocation logic.
> 
> ct_vm_map() always uses PTEs in vm->ptp[0].area regardless of
> CT_PTP_NUM. On AMD64 systems, a single PTP covers 512 PTEs (2M). When
> aggregate memory allocations exceed this limit, ct_vm_map() tries to
> access beyond the allocated space and causes a page fault:
> 
>   BUG: unable to handle page fault for address: ffffd4ae8a10a000
>   Oops: Oops: 0002 [#1] SMP PTI
>   RIP: 0010:ct_vm_map+0x17c/0x280 [snd_ctxfi]
>   Call Trace:
>   atc_pcm_playback_prepare+0x225/0x3b0
>   ct_pcm_playback_prepare+0x38/0x60
>   snd_pcm_do_prepare+0x2f/0x50
>   snd_pcm_action_single+0x36/0x90
>   snd_pcm_action_nonatomic+0xbf/0xd0
>   snd_pcm_ioctl+0x28/0x40
>   __x64_sys_ioctl+0x97/0xe0
>   do_syscall_64+0x81/0x610
>   entry_SYSCALL_64_after_hwframe+0x76/0x7e
> 
> Revert CT_PTP_NUM to 1. The 256 SRC_RESOURCE_NUM and playback_count
> remain unchanged.
> 
> Fixes: 391e69143d0a ("ALSA: ctxfi: Bump playback substreams to 256")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Harin Lee <me@xxxxxxxxx>

Applied to for-next branch now, as it's no regression in 7.0, per se.

But, I wonder whether fixing in ctvmem.c would be relatively easy.
Isn't it only about ct_vm_map() and ct_get_ptp_phys()?


thanks,

Takashi