Re: boot failure because of inaccurate page_pool_page_is_pp() on 32-bit kernels

[David Hildenbrand <david@redhat.com>]

On 12.09.25 00:12, Helge Deller wrote:
> As reported earlier in this mail thread, all 32-bit Linux kernels since v6.16
> fail to boot on the parisc architecture like this:
> 
>   BUG: Bad page state in process swapper  pfn:000f7
>   page: refcount:0 mapcount:0 mapping:00000000 index:0x0 pfn:0xf7
>   flags: 0x0(zone=0)
>   raw: 00000000 118022c0 118022c0 00000000 00000000 00000000 ffffffff 00000000
>   raw: 00000000
>   page dumped because: page_pool leak
>   Modules linked in:
>   CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 6.15.0-rc1-32bit+ #2730 NONE
>   Hardware name: 9000/778/B160L
>   Backtrace:
>    [<106ece88>] bad_page+0x14c/0x17c
>    [<10406c50>] free_page_is_bad.part.0+0xd4/0xec
>    [<106ed180>] free_page_is_bad+0x80/0x88
>    [<106ef05c>] __free_pages_ok+0x374/0x508
>    [<1011d34c>] __free_pages_core+0x1f0/0x218
>    [<1011a2f0>] memblock_free_pages+0x68/0x94
>    [<10120324>] memblock_free_all+0x26c/0x310
>    [<1011a4d8>] mm_core_init+0x18c/0x208
>    [<10100e88>] start_kernel+0x4ec/0x7a0
>    [<101054d0>] start_parisc+0xb4/0xc4
> 
> git bisecting leads to this patch which triggers the crash:
> 
>   commit ee62ce7a1d909ccba0399680a03c2dee83bcae95
>   Author: Toke Høiland-Jørgensen <toke@redhat.com>
>   Date:   Wed Apr 9 12:41:37 2025 +0200
>      page_pool: Track DMA-mapped pages and unmap them when destroying the pool
> 
> It turns out that the patch itself isn't wrong.
> 
> But it's the culprit which leads to the kernel bug since it modifies
> PP_MAGIC_MASK for 32-bit kernels from:
> 
> -#define PP_MAGIC_MASK ~0x3UL
> +#define PP_MAGIC_MASK ~(PP_DMA_INDEX_MASK | 0x3UL)
> 
> Function page_pool_page_is_pp() needs to unambiguously identify page pool
> pages (using PP_MAGIC_MASK), but since the patch now reduced the valid bits to
> check in PP_MAGIC_MASK from 0xFFFFFFFC to 0xc000007c, the remaining bits are
> not sufficient to unambiguously identify such pages any longer.
> 
> Because of that, page_pool_page_is_pp() sometimes wrongly reports pages as
> page pool pages and as such triggers the kernel BUG as it believes it found a
> page pool leak.
> 
> IMHO this is a generic 32-bit kernel issue, not just affecting parisc.
> 
> Do you see any options other than:
> a) revert the patch (ee62ce7a1d90), or:
> b) return false in page_pool_page_is_pp() when !defined(CONFIG_64BIT),
>     which means to effectively disable the page pool page test on 32bit
>     machines

We should have a change coming soon that would use a page type and fix 
it as well I think.

https://lkml.kernel.org/r/20250728052742.81294-1-byungchul@sk.com

Until then, the easiest fix would be indeed to go with b).

But maybe the page type thing could be backported?

-- 
Cheers

David / dhildenb