Vlastimil Babka [off-list ref] writes:

On 10/17/25 22:11, Ackerley Tng wrote:

quoted

filemap_add_folio(), called from filemap_grab_folio(), adds folios to
an LRU list. This is unnecessary for guest_memfd, which does not
participate in swapping.

IIRC guest_memfd mappings are unevictable. That should mean they are not
ultimately added to a list (see lruvec_add_folio()).

quoted

In addition, the LRU list takes a reference count on the folio. With

IIUC the refcount is temporary while being on the percpu
&cpu_fbatches.lru_add, added by __folio_batch_add_and_move().

Thanks for pointing this out. You're right about this, I misunderstood
this refcounting earlier.

When flushed
via folio_batch_move_lru(), the refcount is removed and there's only the LRU
folio flag that remains. The fbatch flushing can be triggered if you see an
unexpected refcount increase.

The new plan is, to update kvm_gmem_is_safe_for_conversion() to drain
the fbatch if it some elevated refcount is found:

static bool kvm_gmem_is_safe_for_conversion(struct inode *inode,
					    pgoff_t start, size_t nr_pages,
					    pgoff_t *err_index)
{
	struct address_space *mapping = inode->i_mapping;
	const int filemap_get_folios_refcount = 1;
	pgoff_t last = start + nr_pages - 1;
	struct folio_batch fbatch;
	bool lru_drained = false;
	bool safe = true;
	int i;

	folio_batch_init(&fbatch);
	while (safe && filemap_get_folios(mapping, &start, last, &fbatch)) {

		for (i = 0; i < folio_batch_count(&fbatch);) {
			struct folio *folio = fbatch.folios[i];

			safe = (folio_ref_count(folio) ==
				folio_nr_pages(folio) +
				filemap_get_folios_refcount);

			if (safe) {
				++i;
			} else if (!lru_drained) {
				lru_add_drain_all();
				lru_drained = true;
			} else {
				*err_index = folio->index;
				break;
			}
		}

		folio_batch_release(&fbatch);
	}

	return safe;
}

I hope this is what you meant!

So it might be feasible to do without this
patch (maybe it was already tried and there were substantial issues, in
which case should be mentioned).

The patch "KVM: guest_memfd: Skip LRU for guest_memfd folios" will be
dropped from the next revision, and "KVM: guest_memfd: Don't set
FGP_ACCESSED when getting folios" is no longer a requirement for this
patch series.

quoted

shared-to-private memory conversions for KVM guests dependent on folio
refcounts, this extra reference can cause conversions to fail due to
unexpected refcounts.


[...snip...]