page_pool: Fix PP_MAGIC_MASK to avoid crashing on some 32-bit arches

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>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Tested-by: Helge Deller <deller@gmx.de>
Link: https://patch.msgid.link/20250930114331.675412-1-toke@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions