linux-kernel - [PATCH v2] hugetlb: Fix comments about avoiding atomic allocation of vmemmap pages

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Date:   Tue, 15 Mar 2022 23:16:02 -0400
From:   luofei <luofei@...cloud.com>
To:     <mike.kravetz@...cle.com>, <akpm@...ux-foundation.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        luofei <luofei@...cloud.com>
Subject: [PATCH v2] hugetlb: Fix comments about avoiding atomic allocation of vmemmap pages

Since there is no longer an atomic allocation of vmemmap pages,
but a fixed flag(GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE) is
used. The description of atomicity here is some what inappropriate.

And the atomic parameter naming of update_and_free_page() may
be misleading, add a comment here.

Signed-off-by: luofei <luofei@...cloud.com>
---
 mm/hugetlb.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f8ca7cca3c1a..fbf598bbc4e3 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1569,10 +1569,12 @@ static void __update_and_free_page(struct hstate *h, struct page *page)
 }
 
 /*
- * As update_and_free_page() can be called under any context, so we cannot
- * use GFP_KERNEL to allocate vmemmap pages. However, we can defer the
- * actual freeing in a workqueue to prevent from using GFP_ATOMIC to allocate
- * the vmemmap pages.
+ * Freeing hugetlb pages in done in update_and_free_page(). When freeing
+ * a hugetlb page, vmemmap pages may need to be allocated. The routine
+ * alloc_huge_page_vmemmap() can possibly sleep as it uses GFP_KERNEL.
+ * However, update_and_free_page() can be called under any context. To
+ * avoid the possibility of sleeping in a context where sleeping is not
+ * allowed, defer the actual freeing in a workqueue where sleeping is allowed.
  *
  * free_hpage_workfn() locklessly retrieves the linked list of pages to be
  * freed and frees them one-by-one. As the page->mapping pointer is going
@@ -1616,6 +1618,10 @@ static inline void flush_free_hpage_work(struct hstate *h)
 		flush_work(&free_hpage_work);
 }
 
+/*
+ * atomic == true indicates called from a context where sleeping is
+ * not allowed.
+ */
 static void update_and_free_page(struct hstate *h, struct page *page,
 				 bool atomic)
 {
@@ -1625,7 +1631,8 @@ static void update_and_free_page(struct hstate *h, struct page *page,
 	}
 
 	/*
-	 * Defer freeing to avoid using GFP_ATOMIC to allocate vmemmap pages.
+	 * Defer freeing to avoid possible sleeping when allocating
+	 * vmemmap pages.
 	 *
 	 * Only call schedule_work() if hpage_freelist is previously
 	 * empty. Otherwise, schedule_work() had been called but the workfn
-- 
2.27.0