Re: [PATCH net v2] page_pool: Fix PP_MAGIC_MASK to avoid crashing on some 32-bit arches

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Helge Deller <deller@gmx.de> writes:

> On 9/30/25 13:43, Toke Høiland-Jørgensen wrote:
>> Helge reported that the introduction of PP_MAGIC_MASK let to crashes on
>> boot on his 32-bit parisc machine. The cause of this is the mask is set
>> too wide, so the page_pool_page_is_pp() incurs false positives which
>> crashes the machine.
>> 
>> Just disabling the check in page_pool_is_pp() will lead to the page_pool
>> code itself malfunctioning; so instead of doing this, this patch changes
>> the define for PP_DMA_INDEX_BITS to avoid mistaking arbitrary kernel
>> pointers for page_pool-tagged pages.
>> 
>> The fix relies on the kernel pointers that alias with the pp_magic field
>> always being above PAGE_OFFSET. With this assumption, we can use the
>> lowest bit of the value of PAGE_OFFSET as the upper bound of the
>> PP_DMA_INDEX_MASK, which should avoid the false positives.
>> 
>> Because we cannot rely on PAGE_OFFSET always being a compile-time
>> constant, nor on it always being >0, we fall back to disabling the
>> dma_index storage when there are not enough bits available. This leaves
>> us in the situation we were in before the patch in the Fixes tag, but
>> only on a subset of architecture configurations. This seems to be the
>> best we can do until the transition to page types in complete for
>> page_pool pages.
>> 
>> v2:
>> - Make sure there's at least 8 bits available and that the PAGE_OFFSET
>>    bit calculation doesn't wrap
>> 
>> Link: https://lore.kernel.org/all/aMNJMFa5fDalFmtn@p100/
>> Fixes: ee62ce7a1d90 ("page_pool: Track DMA-mapped pages and unmap them when destroying the pool")
>> Cc: stable@vger.kernel.org # 6.15+
>> Tested-by: Helge Deller <deller@gmx.de>
>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>> ---
>>   include/linux/mm.h   | 22 +++++++------
>>   net/core/page_pool.c | 76 ++++++++++++++++++++++++++++++--------------
>>   2 files changed, 66 insertions(+), 32 deletions(-)
>
> I tested this v2 patch (the former tested-by was for v1), and v2
> works too:
>
> Tested-by: Helge Deller <deller@gmx.de>

Great, thank you for re-testing! :)

-Toke