On Mon 22-03-21 16:07:29, Mike Kravetz wrote:

On 3/22/21 7:07 AM, Michal Hocko wrote:

quoted

On Fri 19-03-21 15:42:03, Mike Kravetz wrote:

quoted

The routine set_max_huge_pages reduces the number of hugetlb_pages,
by calling free_pool_huge_page in a loop.  It does this as long as
persistent_huge_pages() is above a calculated min_count value.
However, this loop can conditionally drop hugetlb_lock and in some
circumstances free_pool_huge_page can drop hugetlb_lock.  If the
lock is dropped, counters could change the calculated min_count
value may no longer be valid.

OK, this one looks like a real bug fix introduced by 55f67141a8927.
Unless I am missing something we could release pages which are reserved
already.
 

quoted

The routine try_to_free_low has the same issue.

Recalculate min_count in each loop iteration as hugetlb_lock may have
been dropped.

Signed-off-by: Mike Kravetz <redacted>
---
 mm/hugetlb.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d5be25f910e8..c537274c2a38 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c

@@ -2521,11 +2521,20 @@ static void __init report_hugepages(void)
 	}
 }
 
+static inline unsigned long min_hp_count(struct hstate *h, unsigned long count)
+{
+	unsigned long min_count;
+
+	min_count = h->resv_huge_pages + h->nr_huge_pages - h->free_huge_pages;
+	return max(count, min_count);

Just out of curiousity, is compiler allowed to inline this piece of code
and then cache the value? In other words do we need to make these
READ_ONCE or otherwise enforce the no-caching behavior?

I honestly do not know if the compiler is allowed to do that.  The
assembly code generated by my compiler does not cache the value, but
that does not guarantee anything.  I can add READ_ONCE to make the
function look something like:

static inline unsigned long min_hp_count(struct hstate *h, unsigned long count)
{
	unsigned long min_count;

	min_count = READ_ONCE(h->resv_huge_pages) + READ_ONCE(h->nr_huge_pages)
					- READ_ONCE(h->free_huge_pages);
	return max(count, min_count);
}

Maybe just forcing to never inline the function should be sufficient.
This is not a hot path to micro optimize for no function call. But there
are much more qualified people on the CC list on this matter who could
clarify. Peter?
-- 
Michal Hocko
SUSE Labs