Re: [External] Re: [PATCH v21 0/9] Free some vmemmap pages of HugeTLB page

From: Mike Kravetz <mike.kravetz@oracle.com>
To: Muchun Song <songmuchun@bytedance.com>
Cc: "Jonathan Corbet" <corbet@lwn.net>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	bp@alien8.de, "X86 ML" <x86@kernel.org>,
	hpa@zytor.com, dave.hansen@linux.intel.com, luto@kernel.org,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	paulmck@kernel.org, pawan.kumar.gupta@linux.intel.com,
	"Randy Dunlap" <rdunlap@infradead.org>,
	oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de,
	"Mina Almasry" <almasrymina@google.com>,
	"David Rientjes" <rientjes@google.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Oscar Salvador" <osalvador@suse.de>,
	"Michal Hocko" <mhocko@suse.com>,
	"Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>,
	"David Hildenbrand" <david@redhat.com>,
	"HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>,
	"Joao Martins" <joao.m.martins@oracle.com>,
	"Xiongchun duan" <duanxiongchun@bytedance.com>,
	fam.zheng@bytedance.com, zhengqi.arch@bytedance.com,
	linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	"Linux Memory Management List" <linux-mm@kvack.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [External] Re: [PATCH v21 0/9] Free some vmemmap pages of HugeTLB page
Date: Wed, 28 Apr 2021 19:31:50 -0700	[thread overview]
Message-ID: <98f191e8-b509-e541-9d9d-76029c74d241@oracle.com> (raw)
In-Reply-To: <CAMZfGtVbB6YwUMg2ECpdmniQ_vt_3AwdVAuu0GdUJfzWZgQpyg@mail.gmail.com>

On 4/28/21 5:26 AM, Muchun Song wrote:
> On Wed, Apr 28, 2021 at 7:47 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>>
>> Thanks!  I will take a look at the modifications soon.
>>
>> I applied the patches to Andrew's mmotm-2021-04-21-23-03, ran some tests and
>> got the following warning.  We may need to special case that call to
>> __prep_new_huge_page/free_huge_page_vmemmap from alloc_and_dissolve_huge_page
>> as it is holding hugetlb lock with IRQs disabled.
> 
> Good catch. Thanks Mike. I will fix it in the next version. How about this:
> 
> @@ -1618,7 +1617,8 @@ static void __prep_new_huge_page(struct hstate
> *h, struct page *page)
> 
>  static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
>  {
> +       free_huge_page_vmemmap(h, page);
>         __prep_new_huge_page(page);
>         spin_lock_irq(&hugetlb_lock);
>         __prep_account_new_huge_page(h, nid);
>         spin_unlock_irq(&hugetlb_lock);
> @@ -2429,6 +2429,7 @@ static int alloc_and_dissolve_huge_page(struct
> hstate *h, struct page *old_page,
>         if (!new_page)
>                 return -ENOMEM;
> 
> +       free_huge_page_vmemmap(h, new_page);
>  retry:
>         spin_lock_irq(&hugetlb_lock);
>         if (!PageHuge(old_page)) {
> @@ -2489,7 +2490,7 @@ static int alloc_and_dissolve_huge_page(struct
> hstate *h, struct page *old_page,
> 
>  free_new:
>         spin_unlock_irq(&hugetlb_lock);
> -       __free_pages(new_page, huge_page_order(h));
> +       update_and_free_page(h, new_page, false);
> 
>         return ret;
>  }
> 
> 

Another option would be to leave the prep* routines as is and only
modify alloc_and_dissolve_huge_page as follows:

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9c617c19fc18..f8e5013a6b46 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2420,14 +2420,15 @@ static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
 
 	/*
 	 * Before dissolving the page, we need to allocate a new one for the
-	 * pool to remain stable. Using alloc_buddy_huge_page() allows us to
-	 * not having to deal with prep_new_huge_page() and avoids dealing of any
-	 * counters. This simplifies and let us do the whole thing under the
-	 * lock.
+	 * pool to remain stable.  Here, we allocate the page and 'prep' it
+	 * by doing everything but actually updating counters and adding to
+	 * the pool.  This simplifies and let us do most of the processing
+	 * under the lock.
 	 */
 	new_page = alloc_buddy_huge_page(h, gfp_mask, nid, NULL, NULL);
 	if (!new_page)
 		return -ENOMEM;
+	__prep_new_huge_page(h, new_page);
 
 retry:
 	spin_lock_irq(&hugetlb_lock);
@@ -2473,7 +2474,6 @@ static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
 		 * Reference count trick is needed because allocator gives us
 		 * referenced page but the pool requires pages with 0 refcount.
 		 */
-		__prep_new_huge_page(h, new_page);
 		__prep_account_new_huge_page(h, nid);
 		page_ref_dec(new_page);
 		enqueue_huge_page(h, new_page);
@@ -2489,7 +2489,7 @@ static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
 
 free_new:
 	spin_unlock_irq(&hugetlb_lock);
-	__free_pages(new_page, huge_page_order(h));
+	update_and_free_page(h, old_page, false);
 
 	return ret;
 }

-- 
Mike Kravetz
