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

[Catalin Marinas <catalin.marinas@arm.com>]

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).

-- 
Catalin