Re: [RFC] mm/page_isolation: Fix an infinite loop in isolate_single_pageblock()

[Zi Yan <ziy@nvidia.com>]

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On 30 May 2022, at 7:50, Anshuman Khandual wrote:

> HugeTLB allocation (32MB pages on 4K base page) via sysfs on arm64 platform
> is getting stuck in isolate_single_pageblock(), because of an infinite loop
> Because head_pfn always evaluate the same, so does pfn, and the outer loop
> never exits. Dropping the relevant code block, which seems redundant, makes
> the problem go away.

Thanks for the report.

>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Fixes: b2c9e2fbba32 ("mm: make alloc_contig_range work at pageblock granularity")
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> I am not sure about this fix, and also did not find much time today to
> debug any further. There are much code changes around this function in
> recent days. This problem is present on latest mainline kernel.
>
> - Anshuman
>
>  mm/page_isolation.c | 4 ----
>  1 file changed, 4 deletions(-)
>
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 6021f8444b5a..b0922fee75c1 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -389,10 +389,6 @@ static int isolate_single_pageblock(unsigned long boundary_pfn, int flags,
>  			struct page *head = compound_head(page);
>  			unsigned long head_pfn = page_to_pfn(head);
>
> -			if (head_pfn + nr_pages <= boundary_pfn) {
> -				pfn = head_pfn + nr_pages;
> -				continue;
> -			}
>  #if defined CONFIG_COMPACTION || defined CONFIG_CMA
>  			/*
>  			 * hugetlb, lru compound (THP), and movable compound pages
> -- 
> 2.20.1

Can you try the patch below to see if it fixes the issue? Thanks.

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index 6021f8444b5a..d200d41ad0d3 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -385,9 +385,9 @@ static int isolate_single_pageblock(unsigned long boundary_pfn, int flags,
                 * above do the rest. If migration is not possible, just fail.
                 */
                if (PageCompound(page)) {
-                       unsigned long nr_pages = compound_nr(page);
                        struct page *head = compound_head(page);
                        unsigned long head_pfn = page_to_pfn(head);
+                       unsigned long nr_pages = compound_nr(head);

                        if (head_pfn + nr_pages <= boundary_pfn) {
                                pfn = head_pfn + nr_pages;


--
Best Regards,
Yan, Zi

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