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SubjectRe: [PATCH v9 12/21] mm: pagewalk: Allow walking without vma
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On 07/22/2019 09:12 PM, Steven Price wrote:
> Since 48684a65b4e3: "mm: pagewalk: fix misbehavior of walk_page_range
> for vma(VM_PFNMAP)", page_table_walk() will report any kernel area as
> a hole, because it lacks a vma.
>
> This means each arch has re-implemented page table walking when needed,
> for example in the per-arch ptdump walker.
>
> Remove the requirement to have a vma except when trying to split huge
> pages.
>
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
> mm/pagewalk.c | 25 +++++++++++++++++--------
> 1 file changed, 17 insertions(+), 8 deletions(-)
>
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index 98373a9f88b8..1cbef99e9258 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -36,7 +36,7 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
> do {
> again:
> next = pmd_addr_end(addr, end);
> - if (pmd_none(*pmd) || !walk->vma) {
> + if (pmd_none(*pmd)) {
> if (walk->pte_hole)
> err = walk->pte_hole(addr, next, walk);
> if (err)
> @@ -59,9 +59,14 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
> if (!walk->pte_entry)
> continue;
>
> - split_huge_pmd(walk->vma, pmd, addr);
> - if (pmd_trans_unstable(pmd))
> - goto again;
> + if (walk->vma) {
> + split_huge_pmd(walk->vma, pmd, addr);

Check for a PMD THP entry before attempting to split it ?

> + if (pmd_trans_unstable(pmd))
> + goto again;
> + } else if (pmd_leaf(*pmd)) {
> + continue;
> + }
> +
> err = walk_pte_range(pmd, addr, next, walk);
> if (err)
> break;
> @@ -81,7 +86,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
> do {
> again:
> next = pud_addr_end(addr, end);
> - if (pud_none(*pud) || !walk->vma) {
> + if (pud_none(*pud)) {
> if (walk->pte_hole)
> err = walk->pte_hole(addr, next, walk);
> if (err)
> @@ -95,9 +100,13 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
> break;
> }
>
> - split_huge_pud(walk->vma, pud, addr);
> - if (pud_none(*pud))
> - goto again;
> + if (walk->vma) {
> + split_huge_pud(walk->vma, pud, addr);

Check for a PUD THP entry before attempting to split it ?

> + if (pud_none(*pud))
> + goto again;
> + } else if (pud_leaf(*pud)) {
> + continue;
> + }

This is bit cryptic. walk->vma check should be inside a helper is_user_page_table()
or similar to make things clear. p4d_leaf() check missing in walk_p4d_range() for
kernel page table walk ? Wondering if p?d_leaf() test should be moved earlier while
calling p?d_entry() for kernel page table walk.

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