On Thu 07-01-21 17:01:16, Muchun Song wrote:

On Thu, Jan 7, 2021 at 4:39 PM Michal Hocko [off-list ref] wrote:

quoted

On Thu 07-01-21 11:11:41, Muchun Song wrote:

quoted

On Thu, Jan 7, 2021 at 1:07 AM Michal Hocko [off-list ref] wrote:

quoted

On Wed 06-01-21 16:47:37, Muchun Song wrote:

quoted

When dissolve_free_huge_page() races with __free_huge_page(), we can
do a retry. Because the race window is small.

Is this a bug fix or mere optimization. I have hard time to tell from
the description.

It is optimization. Thanks.

quoted

quoted

Signed-off-by: Muchun Song <redacted>
---
 mm/hugetlb.c | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c

[...]

quoted

@@ -1825,6 +1828,14 @@ int dissolve_free_huge_page(struct page *page)
      }
 out:
      spin_unlock(&hugetlb_lock);
+
+     /*
+      * If the freeing of the HugeTLB page is put on a work queue, we should
+      * flush the work before retrying.
+      */
+     if (unlikely(rc == -EAGAIN))
+             flush_work(&free_hpage_work);

Is it safe to wait for the work to finish from this context?

Yes. It is safe.

Please expand on why in the changelog. Same for the optimization
including some numbers showing it really helps.

OK. Changelog should be updated. Do you agree the race window
is quite small?

Yes, the race is very rare and the window itself should be relatively
small. This doesn't really answer the question about blocking though.

If so, why is it not an optimization? Don’t we dissolve
the page as successfully as possible when we call
dissolve_free_huge_page()? I am confused about numbers showing.
Because this is not a performance optimization, but an increase in
the success rate of dissolving.

And it is a very theoretical one, right? Can you even trigger it? What
would happen if the race is lost? Is it serious? This all would be part
of the changelog ideally. This is a tricky area that spans hugetlb,
memory hotplug and hwpoisoning. All of them tricky. 
-- 
Michal Hocko
SUSE Labs