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

[David Hildenbrand]

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@xxxxxxxxxx>
> > > ---
> > >    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