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Message-ID: <9f9d7e96-b135-4830-b528-37418ae7bbfd@redhat.com>
Date: Wed, 28 Aug 2024 09:44:04 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Cc: Gavin Shan <gshan@...hat.com>, Catalin Marinas <catalin.marinas@....com>,
x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Thomas Gleixner <tglx@...utronix.de>, Alistair Popple <apopple@...dia.com>,
kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
Sean Christopherson <seanjc@...gle.com>, Oscar Salvador <osalvador@...e.de>,
Jason Gunthorpe <jgg@...dia.com>, Borislav Petkov <bp@...en8.de>,
Zi Yan <ziy@...dia.com>, Axel Rasmussen <axelrasmussen@...gle.com>,
Yan Zhao <yan.y.zhao@...el.com>, Will Deacon <will@...nel.org>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH v2 06/19] mm/pagewalk: Check pfnmap for folio_walk_start()
On 26.08.24 22:43, Peter Xu wrote:
> Teach folio_walk_start() to recognize special pmd/pud mappings, and fail
> them properly as it means there's no folio backing them.
>
> Cc: David Hildenbrand <david@...hat.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
> mm/pagewalk.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index cd79fb3b89e5..12be5222d70e 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -753,7 +753,7 @@ struct folio *folio_walk_start(struct folio_walk *fw,
> fw->pudp = pudp;
> fw->pud = pud;
>
> - if (!pud_present(pud) || pud_devmap(pud)) {
> + if (!pud_present(pud) || pud_devmap(pud) || pud_special(pud)) {
> spin_unlock(ptl);
> goto not_found;
> } else if (!pud_leaf(pud)) {
> @@ -783,7 +783,7 @@ struct folio *folio_walk_start(struct folio_walk *fw,
> fw->pmdp = pmdp;
> fw->pmd = pmd;
>
> - if (pmd_none(pmd)) {
> + if (pmd_none(pmd) || pmd_special(pmd)) {
> spin_unlock(ptl);
> goto not_found;
> } else if (!pmd_leaf(pmd)) {
As raised, this is not the right way to to it. You should follow what
CONFIG_ARCH_HAS_PTE_SPECIAL and vm_normal_page() does.
It's even spelled out in vm_normal_page_pmd() that at the time it was
introduced there was no pmd_special(), so there was no way to handle that.
diff --git a/mm/memory.c b/mm/memory.c
index f0cf5d02b4740..272445e9db147 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -672,15 +672,29 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
{
unsigned long pfn = pmd_pfn(pmd);
- /*
- * There is no pmd_special() but there may be special pmds, e.g.
- * in a direct-access (dax) mapping, so let's just replicate the
- * !CONFIG_ARCH_HAS_PTE_SPECIAL case from vm_normal_page() here.
- */
+ if (IS_ENABLED(CONFIG_ARCH_HAS_PMD_SPECIAL)) {
+ if (likely(!pmd_special(pmd)))
+ goto check_pfn;
+ if (vma->vm_ops && vma->vm_ops->find_special_page)
+ return vma->vm_ops->find_special_page(vma, addr);
+ if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
+ return NULL;
+ if (is_huge_zero_pmd(pmd))
+ return NULL;
+ if (pmd_devmap(pmd))
+ /* See vm_normal_page() */
+ return NULL;
+ return NULL;
+ }
+
+ /* !CONFIG_ARCH_HAS_PMD_SPECIAL case follows: */
+
if (unlikely(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP))) {
if (vma->vm_flags & VM_MIXEDMAP) {
if (!pfn_valid(pfn))
return NULL;
+ if (is_huge_zero_pmd(pmd))
+ return NULL;
goto out;
} else {
unsigned long off;
@@ -692,6 +706,11 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
}
}
+ /*
+ * For historical reasons, these might not have pmd_special() set,
+ * so we'll check them manually, in contrast to vm_normal_page().
+ */
+check_pfn:
if (pmd_devmap(pmd))
return NULL;
if (is_huge_zero_pmd(pmd))
We should then look into mapping huge zeropages also with pmd_special.
pmd_devmap we'll leave alone until removed. But that's indeoendent of your series.
I wonder if CONFIG_ARCH_HAS_PTE_SPECIAL is sufficient and we don't need additional
CONFIG_ARCH_HAS_PMD_SPECIAL.
As I said, if you need someone to add vm_normal_page_pud(), I can handle that.
--
Cheers,
David / dhildenb
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