Re: [PATCH v11 0/6] Use pageblock_order for cma and alloc_contig_range alignment.

[Qian Cai]

On Thu, May 19, 2022 at 05:35:15PM -0400, Zi Yan wrote:
> Do you have a complete reproducer? From your printout, it is clear that a 512-page compound
> page caused the infinite loop, because the page was not migrated and the code kept
> retrying. But __alloc_contig_migrate_range() is supposed to return non-zero to tell the
> code the page cannot be migrated and the code will goto failed without retrying. It will be
> great you can share what exactly has run after boot, so that I can reproduce locally to
> identify what makes __alloc_contig_migrate_range() return 0 without migrating the page.

The reproducer is just to run the same script I shared with you previously
multiple times instead. It is still quite reproducible here as it usually
happens within a hour.

$ for i in `seq 1 100`; do ./flip_mem.py; done

> Can you also try the patch below to see if it fixes the infinite loop?
> 
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index b3f074d1682e..abde1877bbcb 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -417,10 +417,9 @@ static int isolate_single_pageblock(unsigned long boundary_pfn, gfp_t gfp_flags,
>                                 order = 0;
>                                 outer_pfn = pfn;
>                                 while (!PageBuddy(pfn_to_page(outer_pfn))) {
> -                                       if (++order >= MAX_ORDER) {
> -                                               outer_pfn = pfn;
> -                                               break;
> -                                       }
> +                                       /* abort if the free page cannot be found */
> +                                       if (++order >= MAX_ORDER)
> +                                               goto failed;
>                                         outer_pfn &= ~0UL << order;
>                                 }
>                                 pfn = outer_pfn;
> 

Can you explain a bit how this patch is the right thing to do here? I am a
little bit worry about shooting into the dark. Otherwise, I'll be running
the off-by-one part over the weekend to see if that helps.