Re: [PATCH] mm, memory_hotplug: teach has_unmovable_pages about of LRU migrateable pages

[Michal Hocko]

On Mon 05-11-18 10:14:07, Michal Hocko wrote:
> Maybe we can add a retry for movable zone pages.

Or something like this. Ugly as hell, no question about that. I also
have to think about this some more to convince myself this will not
result in an endless loop under some situations.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 48ceda313332..342d66eca0f3 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7779,12 +7779,16 @@ bool has_unmovable_pages(struct zone *zone, struct page 
*page, int count,
        pfn = page_to_pfn(page);
        for (found = 0, iter = 0; iter < pageblock_nr_pages; iter++) {
                unsigned long check = pfn + iter;
+               unsigned long saved_flags;
 
                if (!pfn_valid_within(check))
                        continue;
 
                page = pfn_to_page(check);
 
+retry:
+               saved_flags = READ_ONCE(page->flags);
+
                if (PageReserved(page))
                        goto unmovable;
 
@@ -7840,6 +7844,13 @@ bool has_unmovable_pages(struct zone *zone, struct page 
*page, int count,
                                page->mapping->a_ops->migratepage)
                        continue;
 
+               /*
+                * We might race with the allocation of the page so retry
+                * if flags have changed.
+                */
+               if (saved_flags != READ_ONCE(page->flags))
+                       goto retry;
+
                /*
                 * If there are RECLAIMABLE pages, we need to check
                 * it.  But now, memory offline itself doesn't call
-- 
Michal Hocko
SUSE Labs