[PATCH 09/19] mm: New follow_pfnmap API

Peter Xu
2024-08-09T16:09:43Z

Introduce a pair of APIs to follow pfn mappings to get entry information.
It's very similar to what follow_pte() does before, but different in that
it recognizes huge pfn mappings.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
include/linux/mm.h | 31 ++++++++++
mm/memory.c | 147 +++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 178 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 90ca84200800..7471302658af 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2374,6 +2374,37 @@ int follow_pte(struct vm_area_struct *vma, unsigned long address,
int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
void *buf, int len, int write);

+struct follow_pfnmap_args {
+ /**
+ * Inputs:
+ * @vma: Pointer to @vm_area_struct struct
+ * @address: the virtual address to walk
+ */
+ struct vm_area_struct *vma;
+ unsigned long address;
+ /**
+ * Internals:
+ *
+ * The caller shouldn't touch any of these.
+ */
+ spinlock_t *lock;
+ pte_t *ptep;
+ /**
+ * Outputs:
+ *
+ * @pfn: the PFN of the address
+ * @pgprot: the pgprot_t of the mapping
+ * @writable: whether the mapping is writable
+ * @special: whether the mapping is a special mapping (real PFN maps)
+ */
+ unsigned long pfn;
+ pgprot_t pgprot;
+ bool writable;
+ bool special;
+};
+int follow_pfnmap_start(struct follow_pfnmap_args *args);
+void follow_pfnmap_end(struct follow_pfnmap_args *args);
+
extern void truncate_pagecache(struct inode *inode, loff_t new);
extern void truncate_setsize(struct inode *inode, loff_t newsize);
void pagecache_isize_extended(struct inode *inode, loff_t from, loff_t to);
diff --git a/mm/memory.c b/mm/memory.c
index 67496dc5064f..2194e0f9f541 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6338,6 +6338,153 @@ int follow_pte(struct vm_area_struct *vma, unsigned long address,
}
EXPORT_SYMBOL_GPL(follow_pte);

+static inline void pfnmap_args_setup(struct follow_pfnmap_args *args,
+ spinlock_t *lock, pte_t *ptep,
+ pgprot_t pgprot, unsigned long pfn_base,
+ unsigned long addr_mask, bool writable,
+ bool special)
+{
+ args->lock = lock;
+ args->ptep = ptep;
+ args->pfn = pfn_base + ((args->address & ~addr_mask) >> PAGE_SHIFT);
+ args->pgprot = pgprot;
+ args->writable = writable;
+ args->special = special;
+}
+
+static inline void pfnmap_lockdep_assert(struct vm_area_struct *vma)
+{
+#ifdef CONFIG_LOCKDEP
+ struct address_space *mapping = vma->vm_file->f_mapping;
+
+ if (mapping)
+ lockdep_assert(lockdep_is_held(&vma->vm_file->f_mapping->i_mmap_rwsem) ||
+ lockdep_is_held(&vma->vm_mm->mmap_lock));
+ else
+ lockdep_assert(lockdep_is_held(&vma->vm_mm->mmap_lock));
+#endif
+}
+
+/**
+ * follow_pfnmap_start() - Look up a pfn mapping at a user virtual address
+ * @args: Pointer to struct @follow_pfnmap_args
+ *
+ * The caller needs to setup args->vma and args->address to point to the
+ * virtual address as the target of such lookup. On a successful return,
+ * the results will be put into other output fields.
+ *
+ * After the caller finished using the fields, the caller must invoke
+ * another follow_pfnmap_end() to proper releases the locks and resources
+ * of such look up request.
+ *
+ * During the start() and end() calls, the results in @args will be valid
+ * as proper locks will be held. After the end() is called, all the fields
+ * in @follow_pfnmap_args will be invalid to be further accessed.
+ *
+ * If the PTE maps a refcounted page, callers are responsible to protect
+ * against invalidation with MMU notifiers; otherwise access to the PFN at
+ * a later point in time can trigger use-after-free.
+ *
+ * Only IO mappings and raw PFN mappings are allowed. The mmap semaphore
+ * should be taken for read, and the mmap semaphore cannot be released
+ * before the end() is invoked.
+ *
+ * This function must not be used to modify PTE content.
+ *
+ * Return: zero on success, -ve otherwise.
+ */
+int follow_pfnmap_start(struct follow_pfnmap_args *args)
+{
+ struct vm_area_struct *vma = args->vma;
+ unsigned long address = args->address;
+ struct mm_struct *mm = vma->vm_mm;
+ spinlock_t *lock;
+ pgd_t *pgdp;
+ p4d_t *p4dp, p4d;
+ pud_t *pudp, pud;
+ pmd_t *pmdp, pmd;
+ pte_t *ptep, pte;
+
+ pfnmap_lockdep_assert(vma);
+
+ if (unlikely(address < vma->vm_start || address >= vma->vm_end))
+ goto out;
+
+ if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
+ goto out;
+retry:
+ pgdp = pgd_offset(mm, address);
+ if (pgd_none(*pgdp) || unlikely(pgd_bad(*pgdp)))
+ goto out;
+
+ p4dp = p4d_offset(pgdp, address);
+ p4d = READ_ONCE(*p4dp);
+ if (p4d_none(p4d) || unlikely(p4d_bad(p4d)))
+ goto out;
+
+ pudp = pud_offset(p4dp, address);
+ pud = READ_ONCE(*pudp);
+ if (pud_none(pud))
+ goto out;
+ if (pud_leaf(pud)) {
+ lock = pud_lock(mm, pudp);
+ if (!unlikely(pud_leaf(pud))) {
+ spin_unlock(lock);
+ goto retry;
+ }
+ pfnmap_args_setup(args, lock, NULL, pud_pgprot(pud),
+ pud_pfn(pud), PUD_MASK, pud_write(pud),
+ pud_special(pud));
+ return 0;
+ }
+
+ pmdp = pmd_offset(pudp, address);
+ pmd = pmdp_get_lockless(pmdp);
+ if (pmd_leaf(pmd)) {
+ lock = pmd_lock(mm, pmdp);
+ if (!unlikely(pmd_leaf(pmd))) {
+ spin_unlock(lock);
+ goto retry;
+ }
+ pfnmap_args_setup(args, lock, NULL, pmd_pgprot(pmd),
+ pmd_pfn(pmd), PMD_MASK, pmd_write(pmd),
+ pmd_special(pmd));
+ return 0;
+ }
+
+ ptep = pte_offset_map_lock(mm, pmdp, address, &lock);
+ if (!ptep)
+ goto out;
+ pte = ptep_get(ptep);
+ if (!pte_present(pte))
+ goto unlock;
+ pfnmap_args_setup(args, lock, ptep, pte_pgprot(pte),
+ pte_pfn(pte), PAGE_MASK, pte_write(pte),
+ pte_special(pte));
+ return 0;
+unlock:
+ pte_unmap_unlock(ptep, lock);
+out:
+ return -EINVAL;
+}
+EXPORT_SYMBOL_GPL(follow_pfnmap_start);
+
+/**
+ * follow_pfnmap_end(): End a follow_pfnmap_start() process
+ * @args: Pointer to struct @follow_pfnmap_args
+ *
+ * Must be used in pair of follow_pfnmap_start(). See the start() function
+ * above for more information.
+ */
+void follow_pfnmap_end(struct follow_pfnmap_args *args)
+{
+ if (args->lock)
+ spin_unlock(args->lock);
+ if (args->ptep)
+ pte_unmap(args->ptep);
+}
+EXPORT_SYMBOL_GPL(follow_pfnmap_end);
+
#ifdef CONFIG_HAVE_IOREMAP_PROT
/**
* generic_access_phys - generic implementation for iomem mmap access
--
2.45.0