Re: [PATCH v2 5/5] KVM: guest_memfd: GUP source pages prior to populating guest memory

[Vishal Annapurve <vannapurve@google.com>]

On Mon, Dec 15, 2025 at 7:36 AM Michael Roth <michael.roth@amd.com> wrote:
>
> Currently the post-populate callbacks handle copying source pages into
> private GPA ranges backed by guest_memfd, where kvm_gmem_populate()
> acquires the filemap invalidate lock, then calls a post-populate
> callback which may issue a get_user_pages() on the source pages prior to
> copying them into the private GPA (e.g. TDX).
>
> This will not be compatible with in-place conversion, where the
> userspace page fault path will attempt to acquire filemap invalidate
> lock while holding the mm->mmap_lock, leading to a potential ABBA
> deadlock[1].
>
> Address this by hoisting the GUP above the filemap invalidate lock so
> that these page faults path can be taken early, prior to acquiring the
> filemap invalidate lock.
>
> It's not currently clear whether this issue is reachable with the
> current implementation of guest_memfd, which doesn't support in-place
> conversion, however it does provide a consistent mechanism to provide
> stable source/target PFNs to callbacks rather than punting to
> vendor-specific code, which allows for more commonality across
> architectures, which may be worthwhile even without in-place conversion.
>
> As part of this change, also begin enforcing that the 'src' argument to
> kvm_gmem_populate() must be page-aligned, as this greatly reduces the
> complexity around how the post-populate callbacks are implemented, and
> since no current in-tree users support using a non-page-aligned 'src'
> argument.
>
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Co-developed-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Co-developed-by: Vishal Annapurve <vannapurve@google.com>
> Signed-off-by: Vishal Annapurve <vannapurve@google.com>
> Signed-off-by: Michael Roth <michael.roth@amd.com>

Tested-By: Vishal Annapurve <vannapurve@google.com>