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SubjectRe: [PATCH v9 13/21] mm: pagewalk: Add test_p?d callbacks
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On 07/22/2019 09:12 PM, Steven Price wrote:
> It is useful to be able to skip parts of the page table tree even when
> walking without VMAs. Add test_p?d callbacks similar to test_walk but
> which are called just before a table at that level is walked. If the
> callback returns non-zero then the entire table is skipped.
>
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
> include/linux/mm.h | 11 +++++++++++
> mm/pagewalk.c | 24 ++++++++++++++++++++++++
> 2 files changed, 35 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index b22799129128..325a1ca6f820 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1447,6 +1447,11 @@ void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
> * value means "do page table walk over the current vma,"
> * and a negative one means "abort current page table walk
> * right now." 1 means "skip the current vma."
> + * @test_pmd: similar to test_walk(), but called for every pmd.
> + * @test_pud: similar to test_walk(), but called for every pud.
> + * @test_p4d: similar to test_walk(), but called for every p4d.
> + * Returning 0 means walk this part of the page tables,
> + * returning 1 means to skip this range.
> * @mm: mm_struct representing the target process of page table walk
> * @vma: vma currently walked (NULL if walking outside vmas)
> * @private: private data for callbacks' usage
> @@ -1471,6 +1476,12 @@ struct mm_walk {
> struct mm_walk *walk);
> int (*test_walk)(unsigned long addr, unsigned long next,
> struct mm_walk *walk);
> + int (*test_pmd)(unsigned long addr, unsigned long next,
> + pmd_t *pmd_start, struct mm_walk *walk);
> + int (*test_pud)(unsigned long addr, unsigned long next,
> + pud_t *pud_start, struct mm_walk *walk);
> + int (*test_p4d)(unsigned long addr, unsigned long next,
> + p4d_t *p4d_start, struct mm_walk *walk);
> struct mm_struct *mm;
> struct vm_area_struct *vma;
> void *private;
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index 1cbef99e9258..6bea79b95be3 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -32,6 +32,14 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
> unsigned long next;
> int err = 0;
>
> + if (walk->test_pmd) {
> + err = walk->test_pmd(addr, end, pmd_offset(pud, 0UL), walk);
> + if (err < 0)
> + return err;
> + if (err > 0)
> + return 0;
> + }

Though this attempts to match semantics with test_walk() and be comprehensive
just wondering what are the real world situations when page walking need to be
aborted based on error condition at a given page table level.

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