Re: [Xen-devel] [PATCH RFC] mm: fix regression with deferred struct page init

[Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>]

On Thu, 2019-06-20 at 11:40 +0200, Juergen Gross wrote:
> Commit 0e56acae4b4dd4a9 ("mm: initialize MAX_ORDER_NR_PAGES at a time
> instead of doing larger sections") is causing a regression on some
> systems when the kernel is booted as Xen dom0.
> 
> The system will just hang in early boot.
> 
> Reason is an endless loop in get_page_from_freelist() in case the first
> zone looked at has no free memory. deferred_grow_zone() is always
> returning true due to the following code snipplet:
> 
>   /* If the zone is empty somebody else may have cleared out the zone */
>   if (!deferred_init_mem_pfn_range_in_zone(&i, zone, &spfn, &epfn,
>                                            first_deferred_pfn)) {
>           pgdat->first_deferred_pfn = ULONG_MAX;
>           pgdat_resize_unlock(pgdat, &flags);
>           return true;
>   }
> 
> This in turn results in the loop as get_page_from_freelist() is
> assuming forward progress can be made by doing some more struct page
> initialization.
> 
> Fixes: 0e56acae4b4dd4a9 ("mm: initialize MAX_ORDER_NR_PAGES at a time instead 
> of doing larger sections")
> ---
> This patch makes my system boot again as Xen dom0, but I'm not really
> sure it is the correct way to do it, hence the RFC.
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  mm/page_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d66bc8abe0af..6ee754b5cd92 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1826,7 +1826,7 @@ deferred_grow_zone(struct zone *zone, unsigned int 
> order)
>                                                first_deferred_pfn)) {
>               pgdat->first_deferred_pfn = ULONG_MAX;
>               pgdat_resize_unlock(pgdat, &flags);
> -             return true;
> +             return false;
>       }
>  
>       /*

The one change I might make to this would be to do:
        return first_deferred_pfn != ULONG_MAX;

That way in the event the previous caller did free up the last of the 
pages and empty the zone just before we got here then we will try one more
time. Otherwise if it was already done before we got here we exit.


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