On Fri, Feb 21, 2025 at 12:32 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Thu, Feb 20, 2025 at 10:21:01PM +1300, Barry Song wrote:
> > 2. 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
