Re: [PATCH] mm: avoid livelock on !__GFP_FS allocations

[Pekka Enberg]

On Tue, Oct 25, 2011 at 9:39 AM, Colin Cross <ccross@xxxxxxxxxxx> wrote:
> Under the following conditions, __alloc_pages_slowpath can loop
> forever:
> gfp_mask & __GFP_WAIT is true
> gfp_mask & __GFP_FS is false
> reclaim and compaction make no progress
> order <= PAGE_ALLOC_COSTLY_ORDER
>
> These conditions happen very often during suspend and resume,
> when pm_restrict_gfp_mask() effectively converts all GFP_KERNEL
> allocations into __GFP_WAIT.

Why does it do that? Why don't we fix the gfp mask instead?

> The oom killer is not run because gfp_mask & __GFP_FS is false,
> but should_alloc_retry will always return true when order is less
> than PAGE_ALLOC_COSTLY_ORDER.
>
> Fix __alloc_pages_slowpath to skip retrying when oom killer is
> not allowed by the GFP flags, the same way it would skip if the
> oom killer was allowed but disabled.
>
> Signed-off-by: Colin Cross <ccross@xxxxxxxxxxx>
> ---
>
> An alternative patch would add a did_some_progress argument to
> __alloc_pages_may_oom, and remove the checks in
> __alloc_pages_slowpath that require knowledge of when
> __alloc_pages_may_oom chooses to run out_of_memory. If
> did_some_progress was still zero, it would goto nopage whether
> or not __alloc_pages_may_oom was actually called.
>
>  mm/page_alloc.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index fef8dc3..dcd99b3 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2193,6 +2193,10 @@ rebalance:
>                        }
>
>                        goto restart;
> +               } else {
> +                       /* If we aren't going to try the OOM killer, give up */
> +                       if (!(gfp_mask & __GFP_NOFAIL))
> +                               goto nopage;
>                }
>        }

I don't quite understand how __GFP_WAIT is involved here. Which path
is causing the infinite loop?
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