virt: acrn: stop using follow_pfn

[ Upstream commit 1b265da7ea1e1ae997fa119c2846bb389eb39c6b ]

Patch series "remove follow_pfn".

This series open codes follow_pfn in the only remaining caller, although
the code there remains questionable.  It then also moves follow_phys into
the only user and simplifies it a bit.

This patch (of 3):

Switch from follow_pfn to follow_pte so that we can get rid of follow_pfn.
Note that this doesn't fix any of the pre-existing raciness and lack of
permission checking in the code.

Link: https://lkml.kernel.org/r/20240324234542.2038726-1-hch@lst.de
Link: https://lkml.kernel.org/r/20240324234542.2038726-2-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fei Li <fei1.li@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Stable-dep-of: 3d6586008f7b ("drivers/virt/acrn: fix PFNMAP PTE checks in acrn_vm_ram_map()")
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/virt/acrn/mm.c

@@ -171,18 +171,24 @@ int acrn_vm_ram_map(struct acrn_vm *vm, struct acrn_vm_memmap *memmap)
 	mmap_read_lock(current->mm);
 	vma = vma_lookup(current->mm, memmap->vma_base);
 	if (vma && ((vma->vm_flags & VM_PFNMAP) != 0)) {
+		spinlock_t *ptl;
+		pte_t *ptep;
+
 		if ((memmap->vma_base + memmap->len) > vma->vm_end) {
 			mmap_read_unlock(current->mm);
 			return -EINVAL;
 		}
 
-		ret = follow_pfn(vma, memmap->vma_base, &pfn);
-		mmap_read_unlock(current->mm);
+		ret = follow_pte(vma->vm_mm, memmap->vma_base, &ptep, &ptl);
 		if (ret < 0) {
+			mmap_read_unlock(current->mm);
 			dev_dbg(acrn_dev.this_device,
 				"Failed to lookup PFN at VMA:%pK.\n", (void *)memmap->vma_base);
 			return ret;
 		}
+		pfn = pte_pfn(ptep_get(ptep));
+		pte_unmap_unlock(ptep, ptl);
+		mmap_read_unlock(current->mm);
 
 		return acrn_mm_region_add(vm, memmap->user_vm_pa,
 			 PFN_PHYS(pfn), memmap->len,