linux-kernel - Re: [PATCH v3 3/5] hugetlb: only set HPageMigratable for migratable hstates

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Message-ID: <62a80585-2a73-10cc-4a2d-5721540d4ad2@oracle.com>
Date:   Fri, 29 Jan 2021 10:46:15 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Oscar Salvador <osalvador@...e.de>, Michal Hocko <mhocko@...e.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Miaohe Lin <linmiaohe@...wei.com>
Subject: Re: [PATCH v3 3/5] hugetlb: only set HPageMigratable for migratable
 hstates

On 1/28/21 2:15 PM, Andrew Morton wrote:
> On Thu, 28 Jan 2021 14:00:29 -0800 Mike Kravetz <mike.kravetz@...cle.com> wrote:
>>
>> Michal suggested that comments describing synchronization be added for each
>> flag.  Since I did 'one patch per flag', that would be an update to each patch.
>> Or, I could simply add a patch to update the comment block based on what you
>> already have.
>>
>> Let me know what is best/easiest for you.
> 
> I guess just one patch is best for reviewers.  Then I'll split up into
> a sprinkle of -fix patches if I'm feeling energetic ;)

Here is a patch to update the comments for all those flags.  It should
apply on top of what is in your tree.

From: Mike Kravetz <mike.kravetz@...cle.com>
Date: Fri, 29 Jan 2021 10:36:12 -0800
Subject: [PATCH] huegtlb: add synchronization information for new hugetlb
 specific flags

Adding comments, no functional change.

Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
---
 include/linux/hugetlb.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index e5e6ffd55392..cf70795d2209 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -480,14 +480,24 @@ unsigned long hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
  * HPG_restore_reserve - Set when a hugetlb page consumes a reservation at
  *	allocation time.  Cleared when page is fully instantiated.  Free
  *	routine checks flag to restore a reservation on error paths.
+ *	Synchronization:  Examined or modified by code that knows it has
+ *	the only reference to page.  i.e. After allocation but before use
+ *	or when the page is being freed.
  * HPG_migratable  - Set after a newly allocated page is added to the page
  *	cache and/or page tables.  Indicates the page is a candidate for
  *	migration.
+ *	Synchronization:  Initially set after new page allocation with no
+ *	locking.  When examined and modified during migration processing
+ *	(isolate, migrate, putback) the hugetlb_lock is held.
  * HPG_temporary - - Set on a page that is temporarily allocated from the buddy
  *	allocator.  Typically used for migration target pages when no pages
  *	are available in the pool.  The hugetlb free page path will
  *	immediately free pages with this flag set to the buddy allocator.
+ *	Synchronization: Can be set after huge page allocation from buddy when
+ *	code knows it has only reference.  All other examinations and
+ *	modifications require hugetlb_lock.
  * HPG_freed - Set when page is on the free lists.
+ *	Synchronization: hugetlb_lock held for examination and modification.
  */
 enum hugetlb_page_flags {
 	HPG_restore_reserve = 0,
-- 
2.29.2