On 18.03.24 11:00, Yin, Fengwei wrote:
On 3/18/2024 10:16 AM, Huang, Ying wrote:
Ryan Roberts <ryan.roberts@xxxxxxx> writes:
Hi Yin Fengwei,
On 15/03/2024 11:12, David Hildenbrand wrote:
On 15.03.24 11:49, Ryan Roberts wrote:
On 15/03/2024 10:43, David Hildenbrand wrote:
On 11.03.24 16:00, Ryan Roberts wrote:
Now that swap supports storing all mTHP sizes, avoid splitting large
folios before swap-out. This benefits performance of the swap-out path
by eliding split_folio_to_list(), which is expensive, and also sets us
up for swapping in large folios in a future series.
If the folio is partially mapped, we continue to split it since we want
to avoid the extra IO overhead and storage of writing out pages
uneccessarily.
Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
---
mm/vmscan.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index cf7d4cf47f1a..0ebec99e04c6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1222,11 +1222,12 @@ static unsigned int shrink_folio_list(struct list_head
*folio_list,
if (!can_split_folio(folio, NULL))
goto activate_locked;
/*
- * Split folios without a PMD map right
- * away. Chances are some or all of the
- * tail pages can be freed without IO.
+ * Split partially mapped folios map
+ * right away. Chances are some or all
+ * of the tail pages can be freed
+ * without IO.
*/
- if (!folio_entire_mapcount(folio) &&
+ if (!list_empty(&folio->_deferred_list) &&
split_folio_to_list(folio,
folio_list))
goto activate_locked;
Not sure if we might have to annotate that with data_race().
I asked that exact question to Matthew in another context bt didn't get a
response. There are examples of checking if the deferred list is empty with and
without data_race() in the code base. But list_empty() is implemented like this:
static inline int list_empty(const struct list_head *head)
{
return READ_ONCE(head->next) == head;
}
So I assumed the READ_ONCE() makes everything safe without a lock? Perhaps not
sufficient for KCSAN?
I don't think READ_ONCE() can replace the lock.
Yeah, there is only one use of data_race with that list.
It was added in f3ebdf042df4 ("THP: avoid lock when check whether THP is in
deferred list").
Looks like that was added right in v1 of that change [1], so my best guess is
that it is not actually required.
If not required, likely we should just cleanup the single user.
[1]
https://lore.kernel.org/linux-mm/20230417075643.3287513-2-fengwei.yin@xxxxxxxxx/
Do you have any recollection of why you added the data_race() markup?
Per my understanding, this is used to mark that the code accesses
folio->_deferred_list without lock intentionally, while
folio->_deferred_list may be changed in parallel. IIUC, this is what
data_race() is used for. Or, my understanding is wrong?
Yes. This is my understanding also.
Why don't we have a data_race() in deferred_split_folio() then, before
taking the lock?
It's used a bit inconsistently here.
--
Cheers,
David / dhildenb
