Hi Vishal,
On Fri, 21 Mar 2025 at 20:09, Vishal Annapurve <vannapurve@xxxxxxxxxx> wrote:
>
> On Tue, Mar 18, 2025 at 9:20 AM Fuad Tabba <tabba@xxxxxxxxxx> wrote:
> > ...
> > +/*
> > + * Callback function for __folio_put(), i.e., called once all references by the
> > + * host to the folio have been dropped. This allows gmem to transition the state
> > + * of the folio to shared with the guest, and allows the hypervisor to continue
> > + * transitioning its state to private, since the host cannot attempt to access
> > + * it anymore.
> > + */
> > void kvm_gmem_handle_folio_put(struct folio *folio)
> > {
> > - WARN_ONCE(1, "A placeholder that shouldn't trigger. Work in progress.");
> > + struct address_space *mapping;
> > + struct xarray *shared_offsets;
> > + struct inode *inode;
> > + pgoff_t index;
> > + void *xval;
> > +
> > + mapping = folio->mapping;
> > + if (WARN_ON_ONCE(!mapping))
> > + return;
> > +
> > + inode = mapping->host;
> > + index = folio->index;
> > + shared_offsets = &kvm_gmem_private(inode)->shared_offsets;
> > + xval = xa_mk_value(KVM_GMEM_GUEST_SHARED);
> > +
> > + filemap_invalidate_lock(inode->i_mapping);
>
> As discussed in the guest_memfd upstream, folio_put can happen from
> atomic context [1], so we need a way to either defer the work outside
> kvm_gmem_handle_folio_put() (which is very likely needed to handle
> hugepages and merge operation) or ensure to execute the logic using
> synchronization primitives that will not sleep.
Thanks for pointing this out. For now, rather than deferring (which
we'll come to when hugepages come into play), I think this would be
possible to resolve by ensuring we have exclusive access* to the folio
instead, and using that to ensure that we can access the
shared_offsets maps.
* By exclusive access I mean either holding the folio lock, or knowing
that no one else has references to the folio (which is the case when
kvm_gmem_handle_folio_put() is called).
I'll try to respin something in time for folks to look at it before
the next sync.
Cheers,
/fuad
> [1] https://elixir.bootlin.com/linux/v6.14-rc6/source/include/linux/mm.h#L1483
>
> > + folio_lock(folio);
> > + kvm_gmem_restore_pending_folio(folio, inode);
> > + folio_unlock(folio);
> > + WARN_ON_ONCE(xa_err(xa_store(shared_offsets, index, xval, GFP_KERNEL)));
> > + filemap_invalidate_unlock(inode->i_mapping);
> > }
> > EXPORT_SYMBOL_GPL(kvm_gmem_handle_folio_put);
> >
> > --
> > 2.49.0.rc1.451.g8f38331e32-goog
> >
