Vlastimil Babka <[email protected]> writes:
> On 10/17/25 22:11, Ackerley Tng wrote:
>> 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()).
>
>> 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.
>> 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...]
>>
