On 18.04.24 16:50, Lance Yang wrote:
Hey David,
FWIW, just a nit below.
Hi!
Thanks, but that was done on purpose.
This way, we'll have a memory barrier (due to at least one
atomic_inc_and_test()) between incrementing the folio refcount
(happening before the rmap change) and incrementing the mapcount.
Is it required? Not 100% sure, refcount vs. mapcount checks are always a
bit racy. But doing it this way let me sleep better at night ;)
[with no subpage mapcounts, we'd do the atomic_inc_and_test on the large
mapcount and have the memory barrier there again; but that's stuff for
the future]
Thanks!
diff --git a/mm/rmap.c b/mm/rmap.c
index 2608c40dffad..08bb6834cf72 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1143,7 +1143,6 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
int *nr_pmdmapped)
{
atomic_t *mapped = &folio->_nr_pages_mapped;
- const int orig_nr_pages = nr_pages;
int first, nr = 0;
__folio_rmap_sanity_checks(folio, page, nr_pages, level);
@@ -1155,6 +1154,7 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
break;
}
+ atomic_add(nr_pages, &folio->_large_mapcount);
do {
first = atomic_inc_and_test(&page->_mapcount);
if (first) {
@@ -1163,7 +1163,6 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
nr++;
}
} while (page++, --nr_pages > 0);
- atomic_add(orig_nr_pages, &folio->_large_mapcount);
break;
case RMAP_LEVEL_PMD:
first = atomic_inc_and_test(&folio->_entire_mapcount);
Thanks,
Lance
--
Cheers,
David / dhildenb
