On Fri, Feb 21, 2025 at 12:32 PM Peter Xu peterx@redhat.com wrote:
On Thu, Feb 20, 2025 at 10:21:01PM +1300, Barry Song wrote:
- src_anon_vma and its lock – swapcache doesn’t require it（folio is not mapped）
Could you help explain what guarantees the rmap walk not happen on a swapcache page?
I'm not familiar with this path, though at least I see damon can start a rmap walk on PageAnon almost with no locking.. some explanations would be appreciated.
I am observing the following in folio_referenced(), which the anon_vma lock was originally intended to protect.
if (!pra.mapcount) return 0;
I assume all other rmap walks should do the same?
int folio_referenced(struct folio *folio, int is_locked, struct mem_cgroup *memcg, unsigned long *vm_flags) {
bool we_locked = false; struct folio_referenced_arg pra = { .mapcount = folio_mapcount(folio), .memcg = memcg, };
struct rmap_walk_control rwc = { .rmap_one = folio_referenced_one, .arg = (void *)&pra, .anon_lock = folio_lock_anon_vma_read, .try_lock = true, .invalid_vma = invalid_folio_referenced_vma, };
*vm_flags = 0; if (!pra.mapcount) return 0; ... }
By the way, since the folio has been under reclamation in this case and isn't in the lru, this should also prevent the rmap walk, right?
-- Peter Xu
Thanks Barry