On Tue, Jun 17, 2025 at 08:12:50AM +0800, Edgecombe, Rick P wrote: > On Mon, 2025-06-16 at 17:59 +0800, Yan Zhao wrote: > > If the above changes are agreeable, we could consider a more ambitious approach: > > introducing an interface like: > > > > int guest_memfd_add_page_ref_count(gfn_t gfn, int nr); > > int guest_memfd_dec_page_ref_count(gfn_t gfn, int nr); > > We talked about doing something like having tdx_hold_page_on_error() in > guestmemfd with a proper name. The separation of concerns will be better if we > can just tell guestmemfd, the page has an issue. Then guestmemfd can decide how > to handle it (refcount or whatever). Instead of using tdx_hold_page_on_error(), the advantage of informing guest_memfd that TDX is holding a page at 4KB granularity is that, even if there is a bug in KVM (such as forgetting to notify TDX to remove a mapping in handle_removed_pt()), guest_memfd would be aware that the page remains mapped in the TDX module. This allows guest_memfd to determine how to handle the problematic page (whether through refcount adjustments or other methods) before truncating it. > > > > This would allow guest_memfd to maintain an internal reference count for each > > private GFN. TDX would call guest_memfd_add_page_ref_count() for mapping and > > guest_memfd_dec_page_ref_count() after a successful unmapping. Before truncating > > a private page from the filemap, guest_memfd could increase the real folio > > reference count based on its internal reference count for the private GFN. > > What does this get us exactly? This is the argument to have less error prone > code that can survive forgetting to refcount on error? I don't see that it is an > especially special case. Yes, for a less error prone code. If this approach is considered too complex for an initial implementation, using tdx_hold_page_on_error() is also a viable option.
