Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list()

From: David Hildenbrand
Date: Mon Mar 18 2024 - 06:05:32 EST

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