Re: [RFC PATCH v11 12/29] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

[Binbin Wu]

On 8/31/2023 12:44 AM, Ackerley Tng wrote:
> Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx> writes:
>
> > > <snip>
> > >
> > > +static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len)
> > > +{
> > > +	struct address_space *mapping = inode->i_mapping;
> > > +	pgoff_t start, index, end;
> > > +	int r;
> > > +
> > > +	/* Dedicated guest is immutable by default. */
> > > +	if (offset + len > i_size_read(inode))
> > > +		return -EINVAL;
> > > +
> > > +	filemap_invalidate_lock_shared(mapping);
> > > +
> > > +	start = offset >> PAGE_SHIFT;
> > > +	end = (offset + len) >> PAGE_SHIFT;
> > > +
> > > +	r = 0;
> > > +	for (index = start; index < end; ) {
> > > +		struct folio *folio;
> > > +
> > > +		if (signal_pending(current)) {
> > > +			r = -EINTR;
> > > +			break;
> > > +		}
> > > +
> > > +		folio = kvm_gmem_get_folio(inode, index);
> > > +		if (!folio) {
> > > +			r = -ENOMEM;
> > > +			break;
> > > +		}
> > > +
> > > +		index = folio_next_index(folio);
> > > +
> > > +		folio_unlock(folio);
> > > +		folio_put(folio);
> > May be a dumb question, why we get the folio and then put it immediately?
> > Will it make the folio be released back to the page allocator?
> I was wondering this too, but it is correct.
>
> In filemap_grab_folio(), the refcount is incremented in three places:
>
> + When the folio is created in filemap_alloc_folio(), it is given a
>    refcount of 1 in
>
>      filemap_alloc_folio() -> folio_alloc() -> __folio_alloc_node() ->
>      __folio_alloc() -> __alloc_pages() -> get_page_from_freelist() ->
>      prep_new_page() -> post_alloc_hook() -> set_page_refcounted()
>
> + Then, in filemap_add_folio(), the refcount is incremented twice:
>
>      + The first is from the filemap (1 refcount per page if this is a
>        hugepage):
>
>          filemap_add_folio() -> __filemap_add_folio() -> folio_ref_add()
>
>      + The second is a refcount from the lru list
>
>          filemap_add_folio() -> folio_add_lru() -> folio_get() ->
>          folio_ref_inc()
>
> In the other path, if the folio exists in the page cache (filemap), the
> refcount is also incremented through
>
>      filemap_grab_folio() -> __filemap_get_folio() -> filemap_get_entry()
>      -> folio_try_get_rcu()
>
> I believe all the branches in kvm_gmem_get_folio() are taking a refcount
> on the folio while the kernel does some work on the folio like clearing
> the folio in clear_highpage() or getting the next index, and then when
> done, the kernel does folio_put().
>
> This pattern is also used in shmem and hugetlb. :)

Thanks for your explanation. It helps a lot.

> I'm not sure whose refcount the folio_put() in kvm_gmem_allocate() is
> dropping though:
>
> + The refcount for the filemap depends on whether this is a hugepage or
>    not, but folio_put() strictly drops a refcount of 1.
> + The refcount for the lru list is just 1, but doesn't the page still
>    remain in the lru list?

I guess the refcount drop here is the one get on the fresh allocation.
Now the filemap has grabbed the folio, so the lifecycle of the folio now 
is decided by the filemap/inode?

> > > +
> > > +		/* 64-bit only, wrapping the index should be impossible. */
> > > +		if (WARN_ON_ONCE(!index))
> > > +			break;
> > > +
> > > +		cond_resched();
> > > +	}
> > > +
> > > +	filemap_invalidate_unlock_shared(mapping);
> > > +
> > > +	return r;
> > > +}
> > > +
> > >
> > > <snip>