linux-kernel - Re: [PATCH] hugetlbfs: fix description about atomic allocation of vmemmap pages when free huge page

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Date:   Tue, 15 Mar 2022 14:16:14 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <songmuchun@...edance.com>,
        luofei <luofei@...cloud.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hugetlbfs: fix description about atomic allocation of
 vmemmap pages when free huge page

On 3/15/22 06:29, Muchun Song wrote:
> On Tue, Mar 15, 2022 at 12:24 PM luofei <luofei@...cloud.com> wrote:
>>
>> No matter what context update_and_free_page() is called in,
>> the flag for allocating the vmemmap page is fixed
>> (GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE), and no atomic
>> allocation is involved, so the description of atomicity here
>> is somewhat inappropriate.
>>
>> and the atomic parameter naming of update_and_free_page() is
>> somewhat misleading.
>>
>> Signed-off-by: luofei <luofei@...cloud.com>
>> ---
>>  mm/hugetlb.c | 10 ++++------
>>  1 file changed, 4 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index f8ca7cca3c1a..239ef82b7897 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -1570,8 +1570,8 @@ 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
>> + * use GFP_ATOMIC to allocate vmemmap pages. However, we can defer the
>> + * actual freeing in a workqueue to prevent waits caused by allocating
>>   * the vmemmap pages.
>>   *
>>   * free_hpage_workfn() locklessly retrieves the linked list of pages to be
>> @@ -1617,16 +1617,14 @@ static inline void flush_free_hpage_work(struct hstate *h)
>>  }
>>
>>  static void update_and_free_page(struct hstate *h, struct page *page,
>> -                                bool atomic)
>> +                                bool delay)
> 
> Hi luofei,
> 
> At least, I don't agree with this change.  The "atomic" means if the
> caller is under atomic context instead of whether using atomic
> GFP_MASK.  The "delay" seems to tell the caller that it can undelay
> the allocation even if it is under atomic context (actually, it has no
> choice).  But "atomic" can indicate the user is being asked to tell us
> if it is under atomic context.

There may be some confusion since GFP_ATOMIC is mentioned in the comments
and GFP_ATOMIC is not used in the allocation of vmemmap pages.  IIRC,
the use of GFP_ATOMIC was discussed at one time but dismissed because of
undesired side effects such as dipping into "atomic reserves".

How about an update to the comments as follows (sorry mailer may mess up
formatting)?

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f8ca7cca3c1a..6a4d27e24b21 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

-- 
Mike Kravetz