linux-kernel - Re: [PATCH] mm/page_isolation: don't putback unisolated page

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Message-ID: <5f7c4ac9-d17a-5af3-0326-1152cca38465@redhat.com>
Date:   Mon, 6 Sep 2021 13:50:10 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     vbabka@...e.cz, iamjoonsoo.kim@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_isolation: don't putback unisolated page

On 06.09.21 13:32, Miaohe Lin wrote:
> On 2021/9/6 17:40, David Hildenbrand wrote:
>> On 04.09.21 11:18, Miaohe Lin wrote:
>>> If __isolate_free_page() failed, due to zone watermark check, the page is
>>> still on the free list. But this page will be put back to free list again
>>> via __putback_isolated_page() now. This may trigger page->flags checks in
>>> __free_one_page() if PageReported is set. Or we will corrupt the free list
>>> because list_add() will be called for pages already on another list.
>>>
>>> Fixes: 3c605096d315 ("mm/page_alloc: restrict max order of merging on isolated pageblock")
>>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>>> ---
>>>    mm/page_isolation.c | 6 ++----
>>>    1 file changed, 2 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>>> index 9bb562d5d194..7d70d772525c 100644
>>> --- a/mm/page_isolation.c
>>> +++ b/mm/page_isolation.c
>>> @@ -93,10 +93,8 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>>>                buddy_pfn = __find_buddy_pfn(pfn, order);
>>>                buddy = page + (buddy_pfn - pfn);
>>>    -            if (!is_migrate_isolate_page(buddy)) {
>>> -                __isolate_free_page(page, order);
>>> -                isolated_page = true;
>>> -            }
>>> +            if (!is_migrate_isolate_page(buddy))
>>> +                isolated_page = !!__isolate_free_page(page, order);
>>>            }
>>>        }
>>>   
>>
>> Shouldn't we much rather force to ignore watermarks here and make sure __isolate_free_page() never fails?
> 
> It seems it is not easy to force to ignore watermarks here. And it's not a problem
> if __isolate_free_page() fails because we can do move_freepages_block() anyway.
> What do you think? Many thanks.

I'm wondering if all this complexity in this function is even required. What about something like this: (completely untested)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index bddf788f45bf..29ff2fcb339c 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -66,12 +66,10 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
  
  static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
  {
+       bool buddy_merge_possible = false;
         struct zone *zone;
         unsigned long flags, nr_pages;
-       bool isolated_page = false;
         unsigned int order;
-       unsigned long pfn, buddy_pfn;
-       struct page *buddy;
  
         zone = page_zone(page);
         spin_lock_irqsave(&zone->lock, flags);
@@ -79,26 +77,15 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
                 goto out;
  
         /*
-        * Because freepage with more than pageblock_order on isolated
-        * pageblock is restricted to merge due to freepage counting problem,
-        * it is possible that there is free buddy page.
-        * move_freepages_block() doesn't care of merge so we need other
-        * approach in order to merge them. Isolation and free will make
-        * these pages to be merged.
+        * If our free page spans at least this whole pageblock and could
+        * eventually get merged into an even bigger page, go via
+        * __putback_isolated_page(), because move_freepages_block() won't
+        * trigger merging of free pages.
          */
         if (PageBuddy(page)) {
                 order = buddy_order(page);
-               if (order >= pageblock_order && order < MAX_ORDER - 1) {
-                       pfn = page_to_pfn(page);
-                       buddy_pfn = __find_buddy_pfn(pfn, order);
-                       buddy = page + (buddy_pfn - pfn);
-
-                       if (pfn_valid_within(buddy_pfn) &&
-                           !is_migrate_isolate_page(buddy)) {
-                               __isolate_free_page(page, order);
-                               isolated_page = true;
-                       }
-               }
+               if (order >= pageblock_order && order < MAX_ORDER - 1)
+                       buddy_merge_possible = true;
         }
  
         /*
@@ -111,12 +98,12 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
          * onlining - just onlined memory won't immediately be considered for
          * allocation.
          */
-       if (!isolated_page) {
+       if (!buddy_merge_possible) {
                 nr_pages = move_freepages_block(zone, page, migratetype, NULL);
                 __mod_zone_freepage_state(zone, nr_pages, migratetype);
         }
         set_pageblock_migratetype(page, migratetype);
-       if (isolated_page)
+       if (buddy_merge_possible)
                 __putback_isolated_page(page, order, migratetype);
         zone->nr_isolate_pageblock--;
  out:



-- 
Thanks,

David / dhildenb