Re: [RFC PATCH v1 03/26] KVM: Add restricted support for mapping guestmem by the host

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On 26.02.24 18:30, Fuad Tabba wrote:
Hi David,

Thank you very much for reviewing this.

On Mon, Feb 26, 2024 at 9:58 AM David Hildenbrand <david@xxxxxxxxxx> wrote:

On 26.02.24 09:58, Fuad Tabba wrote:
Hi David,

On Thu, Feb 22, 2024 at 4:28 PM David Hildenbrand <david@xxxxxxxxxx> wrote:

+static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf)
+     struct folio *folio;
+     folio = kvm_gmem_get_folio(file_inode(vmf->vma->vm_file), vmf->pgoff);
+     if (!folio)
+             return VM_FAULT_SIGBUS;
+     /*
+      * Check if the page is allowed to be faulted to the host, with the
+      * folio lock held to ensure that the check and incrementing the page
+      * count are protected by the same folio lock.
+      */
+     if (!kvm_gmem_isfaultable(vmf)) {
+             folio_unlock(folio);
+             return VM_FAULT_SIGBUS;
+     }
+     vmf->page = folio_file_page(folio, vmf->pgoff);

We won't currently get hugetlb (or even THP) here. It mimics what shmem
would do.

At the moment there isn't hugetlb support in guest_memfd(), and
neither in pKVM. Although we do plan on supporting it.

finish_fault->set_pte_range() will call folio_add_file_rmap_ptes(),
getting the rmap involved.

Do we have some tests in place that make sure that
fallocate(FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE) will properly unmap
the page again (IOW, that the rmap does indeed work?).

I'm not sure if you mean kernel tests, or if I've tested it. There are
guest_memfd() tests for
run. I've also tested it manually with sample programs, and it behaves
as expected.

Can you point me at the existing tests? I'm interested in
mmap()-specific guest_memfd tests.

A test that would make sense to me:

1) Create guest_memfd() and size it to contain at least one page.

2) mmap() it

3) Write some pattern (e.g., all 1's) to the first page using the mmap

4) fallocate(FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE) the first page

5) Make sure reading from the first page using the mmap reads all 0's

IOW, during 4) we properly unmapped (via rmap) and discarded the page,
such that 5) will populate a fresh page in the page cache filled with
0's and map that one.

The existing tests don't test mmap (or rather, they test the inability
to mmap). They do test FALLOC_FL_PUNCH_HOLE. [1]

The tests for mmap() are ones that I wrote myself. I will write a test
like the one you mentioned, and send it with V2, or earlier if you

Thanks, no need to rush. As long as we have some simple test for that scenario at some point, all good!


David / dhildenb

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