Re: [PATCH v3 2/3] arm64: hugetlb: Fix huge_ptep_get_and_clear() for non-present ptes

Ryan Roberts
2025-02-27T09:06:56Z

On 26/02/2025 17:41, Catalin Marinas wrote:
> On Wed, Feb 26, 2025 at 12:06:52PM +0000, Ryan Roberts wrote:
>> arm64 supports multiple huge_pte sizes. Some of the sizes are covered by
>> a single pte entry at a particular level (PMD_SIZE, PUD_SIZE), and some
>> are covered by multiple ptes at a particular level (CONT_PTE_SIZE,
>> CONT_PMD_SIZE). So the function has to figure out the size from the
>> huge_pte pointer. This was previously done by walking the pgtable to
>> determine the level and by using the PTE_CONT bit to determine the
>> number of ptes at the level.
>>
>> But the PTE_CONT bit is only valid when the pte is present. For
>> non-present pte values (e.g. markers, migration entries), the previous
>> implementation was therefore erroneously determining the size. There is
>> at least one known caller in core-mm, move_huge_pte(), which may call
>> huge_ptep_get_and_clear() for a non-present pte. So we must be robust to
>> this case. Additionally the "regular" ptep_get_and_clear() is robust to
>> being called for non-present ptes so it makes sense to follow the
>> behavior.
>>
>> Fix this by using the new sz parameter which is now provided to the
>> function. Additionally when clearing each pte in a contig range, don't
>> gather the access and dirty bits if the pte is not present.
>>
>> An alternative approach that would not require API changes would be to
>> store the PTE_CONT bit in a spare bit in the swap entry pte for the
>> non-present case. But it felt cleaner to follow other APIs' lead and
>> just pass in the size.
>>
>> As an aside, PTE_CONT is bit 52, which corresponds to bit 40 in the swap
>> entry offset field (layout of non-present pte). Since hugetlb is never
>> swapped to disk, this field will only be populated for markers, which
>> always set this bit to 0 and hwpoison swap entries, which set the offset
>> field to a PFN; So it would only ever be 1 for a 52-bit PVA system where
>> memory in that high half was poisoned (I think!). So in practice, this
>> bit would almost always be zero for non-present ptes and we would only
>> clear the first entry if it was actually a contiguous block. That's
>> probably a less severe symptom than if it was always interpreted as 1
>> and cleared out potentially-present neighboring PTEs.
>>
>> Cc: stable@vger.kernel.org
>> Fixes: 66b3923a1a0f ("arm64: hugetlb: add support for PTE contiguous bit")
>> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>>
>> tmp
>> ---
>
> Random "tmp" here, otherwise the patch looks fine (can be removed when
> applying).

Ugh, sorry. That's me squashing in the changes and forgetting to remove git's
default catting of the 2 commit logs.

I'll assume Will can fix this up. If not shout and I'll repost.

Thanks,
Ryan