Re: [PATCH v5 1/5] mm,page_alloc: Bail out earlier on -ENOMEM in alloc_contig_migrate_range

[Michal Hocko <mhocko@suse.com>]

On Wed 17-03-21 12:12:47, Oscar Salvador wrote:
> Currently, __alloc_contig_migrate_range can generate -EINTR, -ENOMEM or -EBUSY,
> and report them down the chain.
> The problem is that when migrate_pages() reports -ENOMEM, we keep going till we
> exhaust all the try-attempts (5 at the moment) instead of bailing out.
> 
> migrate_pages() bails out right away on -ENOMEM because it is considered a fatal
> error. Do the same here instead of keep going and retrying.

I suspect this is not really a real life problem, right? The allocation
would be more costly in the end but this is to be expected under a heavy
memory pressure.

That being said, bailing out early makes sense to me. But now that
you've made me look into the migrate_pages excellent error state reporting
I suspect we have a bug here. Note the 
"Returns the number of pages that were not migrated, or an error code."

but I do not see putback_movable_pages for ret > 0 so it seems we might
leak some pages.

That aside. Now looking at other callers of migrate_pages most of them
do not care about the number of failed pages. The only one which cares
is migrate_pages syscall (do_migrate_pages). I think it would be much
more reasonable to have migrate_pages (kernel function) return error or
0 and make the only caller which cares to count number of failed pages
(e.g. by returning the number of pages from putback_movable_pages).
 
> Signed-off-by: Oscar Salvador <osalvador@suse.de>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Reviewed-by: David Hildenbrand <david@redhat.com>

The patch itself looks reasonable but make sure to mention this is mere
cosmetic change unless there is a real problem fixed by this.
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/page_alloc.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index cfc72873961d..a4f67063b85f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8481,7 +8481,7 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
>  			}
>  			tries = 0;
>  		} else if (++tries == 5) {
> -			ret = ret < 0 ? ret : -EBUSY;
> +			ret = -EBUSY;
>  			break;
>  		}
>  
> @@ -8491,6 +8491,12 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
>  
>  		ret = migrate_pages(&cc->migratepages, alloc_migration_target,
>  				NULL, (unsigned long)&mtc, cc->mode, MR_CONTIG_RANGE);
> +		/*
> +		 * On -ENOMEM, migrate_pages() bails out right away. It is pointless
> +		 * to retry again over this error, so do the same here.
> +		 */
> +		if (ret == -ENOMEM)
> +			break;
>  	}
>  	if (ret < 0) {
>  		putback_movable_pages(&cc->migratepages);
> -- 
> 2.16.3

-- 
Michal Hocko
SUSE Labs