Hi Peter,
On Wed, 12 Feb 2025 at 18:19, Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Tue, Feb 11, 2025 at 12:11:18PM +0000, Fuad Tabba wrote:
> > Before transitioning a guest_memfd folio to unshared, thereby
> > disallowing access by the host and allowing the hypervisor to
> > transition its view of the guest page as private, we need to be
> > sure that the host doesn't have any references to the folio.
> >
> > This patch introduces a new type for guest_memfd folios, which
> > isn't activated in this series but is here as a placeholder and
> > to facilitate the code in the next patch. This will be used in
> > the future to register a callback that informs the guest_memfd
> > subsystem when the last reference is dropped, therefore knowing
> > that the host doesn't have any remaining references.
> >
> > Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
> > ---
> > include/linux/kvm_host.h | 9 +++++++++
> > include/linux/page-flags.h | 17 +++++++++++++++++
> > mm/debug.c | 1 +
> > mm/swap.c | 9 +++++++++
> > virt/kvm/guest_memfd.c | 7 +++++++
> > 5 files changed, 43 insertions(+)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index f34f4cfaa513..8b5f28f6efff 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -2571,4 +2571,13 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
> > struct kvm_pre_fault_memory *range);
> > #endif
> >
> > +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
> > +void kvm_gmem_handle_folio_put(struct folio *folio);
> > +#else
> > +static inline void kvm_gmem_handle_folio_put(struct folio *folio)
> > +{
> > + WARN_ON_ONCE(1);
> > +}
> > +#endif
> > +
> > #endif
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index 6dc2494bd002..734afda268ab 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -933,6 +933,17 @@ enum pagetype {
> > PGTY_slab = 0xf5,
> > PGTY_zsmalloc = 0xf6,
> > PGTY_unaccepted = 0xf7,
> > + /*
> > + * guestmem folios are used to back VM memory as managed by guest_memfd.
> > + * Once the last reference is put, instead of freeing these folios back
> > + * to the page allocator, they are returned to guest_memfd.
> > + *
> > + * For now, guestmem will only be set on these folios as long as they
> > + * cannot be mapped to user space ("private state"), with the plan of
> > + * always setting that type once typed folios can be mapped to user
> > + * space cleanly.
>
> Does it imply that gmem folios can be mapped to userspace at some point?
> It'll be great if you can share more about it, since as of now it looks
> like anything has a page type cannot use the per-page mapcount.
This is the goal of this series. By the end of this series, you can
map gmem folios, as long as they belong to a VM type that allows it.
My other series, which will be rebased on this one, adds the
distinction of memory shared with the host vs memory private to the
guest:
https://lore.kernel.org/all/20250117163001.2326672-1-tabba@xxxxxxxxxx/
That series deals with the mapcount issue, by only applying the type
once the mapcount is 0. We talked about this in the guest_memfd mm
sync, David Hildenbrand mentioned ongoing work to remove the
overlaying of the type with the memcount. That should solve the
problem completely.
> When looking at this, I also found that __folio_rmap_sanity_checks() has
> some folio_test_hugetlb() tests, not sure whether they're prone to be
> changed too e.g. to cover all pages that have a type, so as to cover gmem.
>
> For the longer term, it'll be definitely nice if gmem folios can be
> mapcounted just like normal file folios. It can enable gmem as a backstore
> just like what normal memfd would do, with gmem managing the folios.
That's the plan, I agree.
> > + */
> > + PGTY_guestmem = 0xf8,
> >
> > PGTY_mapcount_underflow = 0xff
> > };
> > @@ -1082,6 +1093,12 @@ FOLIO_TYPE_OPS(hugetlb, hugetlb)
> > FOLIO_TEST_FLAG_FALSE(hugetlb)
> > #endif
> >
> > +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
>
> This seems to only be defined in follow up patches.. so may need some
> adjustments.
It's a configuration option. If you like, I could bring forward the
patch that adds it to the kconfig file.
Thank you,
/fuad
> > +FOLIO_TYPE_OPS(guestmem, guestmem)
> > +#else
> > +FOLIO_TEST_FLAG_FALSE(guestmem)
> > +#endif
> > +
> > PAGE_TYPE_OPS(Zsmalloc, zsmalloc, zsmalloc)
> >
> > /*
> > diff --git a/mm/debug.c b/mm/debug.c
> > index 8d2acf432385..08bc42c6cba8 100644
> > --- a/mm/debug.c
> > +++ b/mm/debug.c
> > @@ -56,6 +56,7 @@ static const char *page_type_names[] = {
> > DEF_PAGETYPE_NAME(table),
> > DEF_PAGETYPE_NAME(buddy),
> > DEF_PAGETYPE_NAME(unaccepted),
> > + DEF_PAGETYPE_NAME(guestmem),
> > };
> >
> > static const char *page_type_name(unsigned int page_type)
> > diff --git a/mm/swap.c b/mm/swap.c
> > index 47bc1bb919cc..241880a46358 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -38,6 +38,10 @@
> > #include <linux/local_lock.h>
> > #include <linux/buffer_head.h>
> >
> > +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
> > +#include <linux/kvm_host.h>
> > +#endif
> > +
> > #include "internal.h"
> >
> > #define CREATE_TRACE_POINTS
> > @@ -101,6 +105,11 @@ static void free_typed_folio(struct folio *folio)
> > case PGTY_hugetlb:
> > free_huge_folio(folio);
> > return;
> > +#endif
> > +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
> > + case PGTY_guestmem:
> > + kvm_gmem_handle_folio_put(folio);
> > + return;
> > #endif
> > default:
> > WARN_ON_ONCE(1);
> > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > index b2aa6bf24d3a..c6f6792bec2a 100644
> > --- a/virt/kvm/guest_memfd.c
> > +++ b/virt/kvm/guest_memfd.c
> > @@ -312,6 +312,13 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
> > return gfn - slot->base_gfn + slot->gmem.pgoff;
> > }
> >
> > +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
> > +void kvm_gmem_handle_folio_put(struct folio *folio)
> > +{
> > + WARN_ONCE(1, "A placeholder that shouldn't trigger. Work in progress.");
> > +}
> > +#endif /* CONFIG_KVM_GMEM_SHARED_MEM */
> > +
> > static struct file_operations kvm_gmem_fops = {
> > .open = generic_file_open,
> > .release = kvm_gmem_release,
> > --
> > 2.48.1.502.g6dc24dfdaf-goog
> >
> >
>
> --
> Peter Xu
>
