Message-ID: <33e8db9a-7749-44ae-a2a6-27f3e6e8a3e0@gmx.de>
Date: Fri, 26 Sep 2025 17:25:00 +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>,
 Mina Almasry <almasrymina@...gle.com>, Jakub Kicinski <kuba@...nel.org>
Cc: Helge Deller <deller@...nel.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] page_pool: Fix PP_MAGIC_MASK to avoid crashing on
 some 32-bit arches

On 9/26/25 13:38, 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 no 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.
> 
> Link: https://lore.kernel.org/all/aMNJMFa5fDalFmtn@p100/
> Fixes: ee62ce7a1d90 ("page_pool: Track DMA-mapped pages and unmap them when destroying the pool")
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
> Sorry for the delay on getting this out. I have only compile-tested it,
> since I don't have any hardware that triggers the original bug. Helge, I'm
> hoping you can take it for a spin?

I can't comment if the patch is otherwise ok, but it does
indeed fixes the boot problem for me, so:

Tested-by: Helge Deller <deller@....de>

Btw, this can easily be tested with qemu:
./qemu-system-hppa -kernel vmlinux -nographic -serial mon:stdio

If the patch is accepted, can you add the CC-stable tag, so that
it gets pushed down to kernel 6.15+ too?

Thank you, Toke!
Helge

> 
>   include/linux/mm.h   | 18 +++++------
>   net/core/page_pool.c | 76 ++++++++++++++++++++++++++++++--------------
>   2 files changed, 62 insertions(+), 32 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 1ae97a0b8ec7..28541cb40f69 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4159,14 +4159,13 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
>    * since this value becomes part of PP_SIGNATURE; meaning we can just use the
>    * space between the PP_SIGNATURE value (without POISON_POINTER_DELTA), and the
>    * lowest bits of POISON_POINTER_DELTA. On arches where POISON_POINTER_DELTA is
> - * 0, we make sure that we leave the two topmost bits empty, as that guarantees
> - * we won't mistake a valid kernel pointer for a value we set, regardless of the
> - * VMSPLIT setting.
> + * 0, we use the lowest bit of PAGE_OFFSET as the boundary if that value is
> + * known at compile-time.
>    *
> - * Altogether, this means that the number of bits available is constrained by
> - * the size of an unsigned long (at the upper end, subtracting two bits per the
> - * above), and the definition of PP_SIGNATURE (with or without
> - * POISON_POINTER_DELTA).
> + * If the value of PAGE_OFFSET is not known at compile time, or if it is too
> + * small to leave some bits available above PP_SIGNATURE, we define the number
> + * of bits to be 0, which turns off the DMA index tracking altogether (see
> + * page_pool_register_dma_index()).
>    */
>   #define PP_DMA_INDEX_SHIFT (1 + __fls(PP_SIGNATURE - POISON_POINTER_DELTA))
>   #if POISON_POINTER_DELTA > 0
> @@ -4175,8 +4174,9 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
>    */
>   #define PP_DMA_INDEX_BITS MIN(32, __ffs(POISON_POINTER_DELTA) - PP_DMA_INDEX_SHIFT)
>   #else
> -/* Always leave out the topmost two; see above. */
> -#define PP_DMA_INDEX_BITS MIN(32, BITS_PER_LONG - PP_DMA_INDEX_SHIFT - 2)
> +/* Constrain to the lowest bit of PAGE_OFFSET if known; see above. */
> +#define PP_DMA_INDEX_BITS ((__builtin_constant_p(PAGE_OFFSET) && PAGE_OFFSET > PP_SIGNATURE) ? \
> +			      MIN(32, __ffs(PAGE_OFFSET) - PP_DMA_INDEX_SHIFT) : 0)
>   #endif
>   
>   #define PP_DMA_INDEX_MASK GENMASK(PP_DMA_INDEX_BITS + PP_DMA_INDEX_SHIFT - 1, \
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 36a98f2bcac3..e224d2145eed 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -472,11 +472,60 @@ page_pool_dma_sync_for_device(const struct page_pool *pool,
>   	}
>   }
>   
> +static int page_pool_register_dma_index(struct page_pool *pool,
> +					netmem_ref netmem, gfp_t gfp)
> +{
> +	int err = 0;
> +	u32 id;
> +
> +	if (unlikely(!PP_DMA_INDEX_BITS))
> +		goto out;
> +
> +	if (in_softirq())
> +		err = xa_alloc(&pool->dma_mapped, &id, netmem_to_page(netmem),
> +			       PP_DMA_INDEX_LIMIT, gfp);
> +	else
> +		err = xa_alloc_bh(&pool->dma_mapped, &id, netmem_to_page(netmem),
> +				  PP_DMA_INDEX_LIMIT, gfp);
> +	if (err) {
> +		WARN_ONCE(err != -ENOMEM, "couldn't track DMA mapping, please report to netdev@");
> +		goto out;
> +	}
> +
> +	netmem_set_dma_index(netmem, id);
> +out:
> +	return err;
> +}
> +
> +static int page_pool_release_dma_index(struct page_pool *pool,
> +				       netmem_ref netmem)
> +{
> +	struct page *old, *page = netmem_to_page(netmem);
> +	unsigned long id;
> +
> +	if (unlikely(!PP_DMA_INDEX_BITS))
> +		return 0;
> +
> +	id = netmem_get_dma_index(netmem);
> +	if (!id)
> +		return -1;
> +
> +	if (in_softirq())
> +		old = xa_cmpxchg(&pool->dma_mapped, id, page, NULL, 0);
> +	else
> +		old = xa_cmpxchg_bh(&pool->dma_mapped, id, page, NULL, 0);
> +	if (old != page)
> +		return -1;
> +
> +	netmem_set_dma_index(netmem, 0);
> +
> +	return 0;
> +}
> +
>   static bool page_pool_dma_map(struct page_pool *pool, netmem_ref netmem, gfp_t gfp)
>   {
>   	dma_addr_t dma;
>   	int err;
> -	u32 id;
>   
>   	/* Setup DMA mapping: use 'struct page' area for storing DMA-addr
>   	 * since dma_addr_t can be either 32 or 64 bits and does not always fit
> @@ -495,18 +544,10 @@ static bool page_pool_dma_map(struct page_pool *pool, netmem_ref netmem, gfp_t g
>   		goto unmap_failed;
>   	}
>   
> -	if (in_softirq())
> -		err = xa_alloc(&pool->dma_mapped, &id, netmem_to_page(netmem),
> -			       PP_DMA_INDEX_LIMIT, gfp);
> -	else
> -		err = xa_alloc_bh(&pool->dma_mapped, &id, netmem_to_page(netmem),
> -				  PP_DMA_INDEX_LIMIT, gfp);
> -	if (err) {
> -		WARN_ONCE(err != -ENOMEM, "couldn't track DMA mapping, please report to netdev@");
> +	err = page_pool_register_dma_index(pool, netmem, gfp);
> +	if (err)
>   		goto unset_failed;
> -	}
>   
> -	netmem_set_dma_index(netmem, id);
>   	page_pool_dma_sync_for_device(pool, netmem, pool->p.max_len);
>   
>   	return true;
> @@ -684,8 +725,6 @@ void page_pool_clear_pp_info(netmem_ref netmem)
>   static __always_inline void __page_pool_release_netmem_dma(struct page_pool *pool,
>   							   netmem_ref netmem)
>   {
> -	struct page *old, *page = netmem_to_page(netmem);
> -	unsigned long id;
>   	dma_addr_t dma;
>   
>   	if (!pool->dma_map)
> @@ -694,15 +733,7 @@ static __always_inline void __page_pool_release_netmem_dma(struct page_pool *poo
>   		 */
>   		return;
>   
> -	id = netmem_get_dma_index(netmem);
> -	if (!id)
> -		return;
> -
> -	if (in_softirq())
> -		old = xa_cmpxchg(&pool->dma_mapped, id, page, NULL, 0);
> -	else
> -		old = xa_cmpxchg_bh(&pool->dma_mapped, id, page, NULL, 0);
> -	if (old != page)
> +	if (page_pool_release_dma_index(pool, netmem))
>   		return;
>   
>   	dma = page_pool_get_dma_addr_netmem(netmem);
> @@ -712,7 +743,6 @@ static __always_inline void __page_pool_release_netmem_dma(struct page_pool *poo
>   			     PAGE_SIZE << pool->p.order, pool->p.dma_dir,
>   			     DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_WEAK_ORDERING);
>   	page_pool_set_dma_addr_netmem(netmem, 0);
> -	netmem_set_dma_index(netmem, 0);
>   }
>   
>   /* Disconnects a page (from a page_pool).  API users can have a need

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