linux-kernel - [mm-unstable PATCH v7 1/8] mm/hugetlb: check gigantic_page_runtime_supported() in return_unused_surplus

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Message-Id: <20220714042420.1847125-2-naoya.horiguchi@linux.dev>
Date:   Thu, 14 Jul 2022 13:24:13 +0900
From:   Naoya Horiguchi <naoya.horiguchi@...ux.dev>
To:     linux-mm@...ck.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Liu Shixin <liushixin2@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Oscar Salvador <osalvador@...e.de>,
        Muchun Song <songmuchun@...edance.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        linux-kernel@...r.kernel.org
Subject: [mm-unstable PATCH v7 1/8] mm/hugetlb: check gigantic_page_runtime_supported() in return_unused_surplus_pages()

From: Naoya Horiguchi <naoya.horiguchi@....com>

I found a weird state of 1GB hugepage pool, caused by the following
procedure:

  - run a process reserving all free 1GB hugepages,
  - shrink free 1GB hugepage pool to zero (i.e. writing 0 to
    /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages), then
  - kill the reserving process.

, then all the hugepages are free *and* surplus at the same time.

  $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
  3
  $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/free_hugepages
  3
  $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/resv_hugepages
  0
  $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/surplus_hugepages
  3

This state is resolved by reserving and allocating the pages then
freeing them again, so this seems not to result in serious problem.
But it's a little surprising (shrinking pool suddenly fails).

This behavior is caused by hstate_is_gigantic() check in
return_unused_surplus_pages(). This was introduced so long ago in 2008
by commit aa888a74977a ("hugetlb: support larger than MAX_ORDER"), and
at that time the gigantic pages were not supposed to be allocated/freed
at run-time.  Now kernel can support runtime allocation/free, so let's
check gigantic_page_runtime_supported() together.

Signed-off-by: Naoya Horiguchi <naoya.horiguchi@....com>
Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>
---
v4 -> v5:
- drop additional gigantic_page_runtime_supported() checks.

v2 -> v3:
- Fixed typo in patch description,
- add !gigantic_page_runtime_supported() check instead of removing
  hstate_is_gigantic() check (suggested by Miaohe and Muchun)
- add a few more !gigantic_page_runtime_supported() check in
  set_max_huge_pages() (by Mike).
---
 mm/hugetlb.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index a4506ed1f1db..cf8ccee7654c 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2432,8 +2432,7 @@ static void return_unused_surplus_pages(struct hstate *h,
 	/* Uncommit the reservation */
 	h->resv_huge_pages -= unused_resv_pages;
 
-	/* Cannot return gigantic pages currently */
-	if (hstate_is_gigantic(h))
+	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
 		goto out;
 
 	/*
-- 
2.25.1