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

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

[Toke Høiland-Jørgensen]

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(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1ae97a0b8ec7..0905eb6b55ec 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 at least 8 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 +4174,13 @@ 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)
+/* Use the lowest bit of PAGE_OFFSET if there's at least 8 bits available; see above */
+#define PP_DMA_INDEX_MIN_OFFSET (1 << (PP_DMA_INDEX_SHIFT + 8))
+#define PP_DMA_INDEX_BITS ((__builtin_constant_p(PAGE_OFFSET) && \
+			    PAGE_OFFSET >= PP_DMA_INDEX_MIN_OFFSET && \
+			    !(PAGE_OFFSET & (PP_DMA_INDEX_MIN_OFFSET - 1))) ? \
+			      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 492728f9e021..1a5edec485f1 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -468,11 +468,60 @@ 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
@@ -491,18 +540,10 @@ 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;
@@ -680,8 +721,6 @@ 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)
@@ -690,15 +729,7 @@ 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);
@@ -708,7 +739,6 @@ 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
-- 
2.51.0

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

[Mina Almasry]

On Tue, Sep 30, 2025 at 4:43 AM Toke Høiland-Jørgensen <toke@redhat.com> 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(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 1ae97a0b8ec7..0905eb6b55ec 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 at least 8 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 +4174,13 @@ 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)
> +/* Use the lowest bit of PAGE_OFFSET if there's at least 8 bits available; see above */
> +#define PP_DMA_INDEX_MIN_OFFSET (1 << (PP_DMA_INDEX_SHIFT + 8))
> +#define PP_DMA_INDEX_BITS ((__builtin_constant_p(PAGE_OFFSET) && \
> +                           PAGE_OFFSET >= PP_DMA_INDEX_MIN_OFFSET && \
> +                           !(PAGE_OFFSET & (PP_DMA_INDEX_MIN_OFFSET - 1))) ? \
> +                             MIN(32, __ffs(PAGE_OFFSET) - PP_DMA_INDEX_SHIFT) : 0)
> +
>  #endif

It took some staring at, but I think I understand this code and it is
correct. This is the critical check, it's making sure that the bits
used by PAGE_OFFSET are not shared with the bits used for the
dma-index:

> +                           !(PAGE_OFFSET & (PP_DMA_INDEX_MIN_OFFSET - 1))) ? \

The following check confused me for a while, but I think I figured it
out. It's checking that the bits used for PAGE_OFFSET are 'higher'
than the bits used for PP_DMA_INDEX:

> +                           PAGE_OFFSET >= PP_DMA_INDEX_MIN_OFFSET && \

And finally this calculation should indeed be the bits we can use (the
empty space between the lsb set by PAGE_OFFSET and the msb set by the
pp magic:

> +                             MIN(32, __ffs(PAGE_OFFSET) - PP_DMA_INDEX_SHIFT) : 0)

AFAIU we should not need the MIN anymore, since that subtraction is
guaranteed to be positive, but that's a nit.

Reviewed-by: Mina Almasry <almasrymina@google.com>

-- 
Thanks,
Mina

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

[Helge Deller]

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>

Thanks!
Helge

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

[Toke Høiland-Jørgensen]

Mina Almasry <almasrymina@google.com> writes:

> On Tue, Sep 30, 2025 at 4:43 AM Toke Høiland-Jørgensen <toke@redhat.com> 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(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 1ae97a0b8ec7..0905eb6b55ec 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 at least 8 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 +4174,13 @@ 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)
>> +/* Use the lowest bit of PAGE_OFFSET if there's at least 8 bits available; see above */
>> +#define PP_DMA_INDEX_MIN_OFFSET (1 << (PP_DMA_INDEX_SHIFT + 8))
>> +#define PP_DMA_INDEX_BITS ((__builtin_constant_p(PAGE_OFFSET) && \
>> +                           PAGE_OFFSET >= PP_DMA_INDEX_MIN_OFFSET && \
>> +                           !(PAGE_OFFSET & (PP_DMA_INDEX_MIN_OFFSET - 1))) ? \
>> +                             MIN(32, __ffs(PAGE_OFFSET) - PP_DMA_INDEX_SHIFT) : 0)
>> +
>>  #endif
>
> It took some staring at, but I think I understand this code and it is
> correct. This is the critical check, it's making sure that the bits
> used by PAGE_OFFSET are not shared with the bits used for the
> dma-index:
>
>> +                           !(PAGE_OFFSET & (PP_DMA_INDEX_MIN_OFFSET - 1))) ? \
>
> The following check confused me for a while, but I think I figured it
> out. It's checking that the bits used for PAGE_OFFSET are 'higher'
> than the bits used for PP_DMA_INDEX:
>
>> +                           PAGE_OFFSET >= PP_DMA_INDEX_MIN_OFFSET && \
>
> And finally this calculation should indeed be the bits we can use (the
> empty space between the lsb set by PAGE_OFFSET and the msb set by the
> pp magic:
>
>> +                             MIN(32, __ffs(PAGE_OFFSET) - PP_DMA_INDEX_SHIFT) : 0)

Yup, exactly! Thanks for walking through it and confirming that the
logic is sound :)

> AFAIU we should not need the MIN anymore, since that subtraction is
> guaranteed to be positive, but that's a nit.

The MIN was originally there to limit how many bits we use for 64-bit
systems that don't set POISON_POINTER_DELTA, since xarray uses a u32 for
the size of the limits. Not sure if such a combination exists in the
real world, but I figure that having it there doesn't hurt in any case.

> Reviewed-by: Mina Almasry <almasrymina@google.com>

Thanks!

-Toke

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

[Toke Høiland-Jørgensen]

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

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

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Tue, 30 Sep 2025 13:43:29 +0200 you 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.
> 
> [...]

Here is the summary with links:
  - [net,v2] page_pool: Fix PP_MAGIC_MASK to avoid crashing on some 32-bit arches
    https://git.kernel.org/netdev/net/c/95920c2ed02b

You are awesome, thank you!
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