On 10.01.25 07:00, Alistair Popple wrote:
Currently to map a DAX page the DAX driver calls vmf_insert_pfn. This creates a special devmap PTE entry for the pfn but does not take a reference on the underlying struct page for the mapping. This is because DAX page refcounts are treated specially, as indicated by the presence of a devmap entry. To allow DAX page refcounts to be managed the same as normal page refcounts introduce vmf_insert_page_mkwrite(). This will take a reference on the underlying page much the same as vmf_insert_page, except it also permits upgrading an existing mapping to be writable if requested/possible. Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx> --- Updates from v2: - Rename function to make not DAX specific - Split the insert_page_into_pte_locked() change into a separate patch. Updates from v1: - Re-arrange code in insert_page_into_pte_locked() based on comments from Jan Kara. - Call mkdrity/mkyoung for the mkwrite case, also suggested by Jan. --- include/linux/mm.h | 2 ++ mm/memory.c | 36 ++++++++++++++++++++++++++++++++++++ 2 files changed, 38 insertions(+) diff --git a/include/linux/mm.h b/include/linux/mm.h index e790298..f267b06 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -3620,6 +3620,8 @@ int vm_map_pages(struct vm_area_struct *vma, struct page **pages, unsigned long num); int vm_map_pages_zero(struct vm_area_struct *vma, struct page **pages, unsigned long num); +vm_fault_t vmf_insert_page_mkwrite(struct vm_fault *vmf, struct page *page, + bool write); vm_fault_t vmf_insert_pfn(struct vm_area_struct *vma, unsigned long addr, unsigned long pfn); vm_fault_t vmf_insert_pfn_prot(struct vm_area_struct *vma, unsigned long addr, diff --git a/mm/memory.c b/mm/memory.c index 8531acb..c60b819 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2624,6 +2624,42 @@ static vm_fault_t __vm_insert_mixed(struct vm_area_struct *vma, return VM_FAULT_NOPAGE; }+vm_fault_t vmf_insert_page_mkwrite(struct vm_fault *vmf, struct page *page,+ bool write) +{ + struct vm_area_struct *vma = vmf->vma; + pgprot_t pgprot = vma->vm_page_prot; + unsigned long pfn = page_to_pfn(page); + unsigned long addr = vmf->address; + int err; + + if (addr < vma->vm_start || addr >= vma->vm_end) + return VM_FAULT_SIGBUS; + + track_pfn_insert(vma, &pgprot, pfn_to_pfn_t(pfn));
I think I raised this before: why is this track_pfn_insert() in here? It only ever does something to VM_PFNMAP mappings, and that cannot possibly be the case here (nothing in VM_PFNMAP is refcounted, ever)?
+ + if (!pfn_modify_allowed(pfn, pgprot)) + return VM_FAULT_SIGBUS;
Why is that required? Why are we messing so much with PFNs? :) Note that x86 does in there /* If it's real memory always allow */ if (pfn_valid(pfn)) return true; See below, when would we ever have a "struct page *" but !pfn_valid() ?
+ + /* + * We refcount the page normally so make sure pfn_valid is true. + */ + if (!pfn_valid(pfn)) + return VM_FAULT_SIGBUS;
Somebody gave us a "struct page", how could the pfn ever by invalid (not have a struct page)?
I think all of the above regarding PFNs should be dropped -- unless I am missing something important.
+ + if (WARN_ON(is_zero_pfn(pfn) && write)) + return VM_FAULT_SIGBUS;
is_zero_page() if you already have the "page". But note that in validate_page_before_insert() we do have a check that allows for conditional insertion of the shared zeropage.
So maybe this hunk is also not required.
+ + err = insert_page(vma, addr, page, pgprot, write); + if (err == -ENOMEM) + return VM_FAULT_OOM; + if (err < 0 && err != -EBUSY) + return VM_FAULT_SIGBUS; + + return VM_FAULT_NOPAGE; +} +EXPORT_SYMBOL_GPL(vmf_insert_page_mkwrite);
-- Cheers, David / dhildenb
