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

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Message-ID: <03029eb0-921a-4e45-ab23-3cb958199085@gmx.de>
Date: Wed, 1 Oct 2025 09:21:47 +0200
From: Helge Deller <deller@....de>
To: Toke Høiland-Jørgensen <toke@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 Ilias Apalodimas <ilias.apalodimas@...aro.org>,
 Jakub Kicinski <kuba@...nel.org>, Mina Almasry <almasrymina@...gle.com>,
 linux-parisc <linux-parisc@...r.kernel.org>
Cc: stable@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, linux-mm@...ck.org, netdev@...r.kernel.org
Subject: Re: [PATCH net v2] page_pool: Fix PP_MAGIC_MASK to avoid crashing on
 some 32-bit arches

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@...r.kernel.org # 6.15+
> Tested-by: Helge Deller <deller@....de>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.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@....de>

Thanks!
Helge