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

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Date:   Wed, 28 Apr 2021 19:31:50 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.de,
        X86 ML <x86@...nel.org>, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>, paulmck@...nel.org,
        pawan.kumar.gupta@...ux.intel.com,
        Randy Dunlap <rdunlap@...radead.org>, oneukum@...e.com,
        anshuman.khandual@....com, jroedel@...e.de,
        Mina Almasry <almasrymina@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        David Hildenbrand <david@...hat.com>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        fam.zheng@...edance.com, zhengqi.arch@...edance.com,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v21 0/9] Free some vmemmap pages of HugeTLB
 page

On 4/28/21 5:26 AM, Muchun Song wrote:
> On Wed, Apr 28, 2021 at 7:47 AM Mike Kravetz <mike.kravetz@...cle.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