Re: [PATCH v4 3/3] mm: warn about illegal __GFP_NOFAIL usage in a more appropriate location and manner

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On Sat, Aug 31, 2024 at 4:29 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> From: Barry Song <v-songbaohua@xxxxxxxx>
>
> Three points for this change:
>
> 1. We should consolidate all warnings in one place. Currently, the
>    order > 1 warning is in the hotpath, while others are in less
>    likely scenarios. Moving all warnings to the slowpath will reduce
>    the overhead for order > 1 and increase the visibility of other
>    warnings.
>
> 2. We currently have two warnings for order: one for order > 1 in
>    the hotpath and another for order > costly_order in the laziest
>    path. I suggest standardizing on order > 1 since it’s been in
>    use for a long time.
>
> 3. We don't need to check for __GFP_NOWARN in this case. __GFP_NOWARN
>    is meant to suppress allocation failure reports, but here we're
>    dealing with bug detection, not allocation failures. So replace
>    WARN_ON_ONCE_GFP by WARN_ON_ONCE.
>
> Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> ---
>  mm/page_alloc.c | 50 ++++++++++++++++++++++++-------------------------
>  1 file changed, 25 insertions(+), 25 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c81ee5662cc7..e790b4227322 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3033,12 +3033,6 @@ struct page *rmqueue(struct zone *preferred_zone,
>  {
>         struct page *page;
>
> -       /*
> -        * We most definitely don't want callers attempting to
> -        * allocate greater than order-1 page units with __GFP_NOFAIL.
> -        */
> -       WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
> -
>         if (likely(pcp_allowed_order(order))) {
>                 page = rmqueue_pcplist(preferred_zone, zone, order,
>                                        migratetype, alloc_flags);
> @@ -4175,6 +4169,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  {
>         bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
>         bool can_compact = gfp_compaction_allowed(gfp_mask);
> +       bool nofail = gfp_mask & __GFP_NOFAIL;
>         const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
>         struct page *page = NULL;
>         unsigned int alloc_flags;
> @@ -4187,6 +4182,25 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>         unsigned int zonelist_iter_cookie;
>         int reserve_flags;
>
> +       if (unlikely(nofail)) {
> +               /*
> +                * We most definitely don't want callers attempting to
> +                * allocate greater than order-1 page units with __GFP_NOFAIL.
> +                */
> +               WARN_ON_ONCE(order > 1);
> +               /*
> +                * Also we don't support __GFP_NOFAIL without __GFP_DIRECT_RECLAIM,
> +                * otherwise, we may result in lockup.
> +                */
> +               WARN_ON_ONCE(!can_direct_reclaim);
> +               /*
> +                * PF_MEMALLOC request from this context is rather bizarre
> +                * because we cannot reclaim anything and only can loop waiting
> +                * for somebody to do a work for us.
> +                */
> +               WARN_ON_ONCE(current->flags & PF_MEMALLOC);

I believe we should add below warning as well:

  WARN_ON_ONCE(gfp_mask & __GFP_NOMEMALLOC);
  WARN_ON_ONCE(gfp_mask & __GFP_NORETRY);
  WARN_ON_ONCE(gfp_mask & __GFP_RETRY_MAYFAIL);
  ...

I'm not sure if that is enough.
__GFP_NOFAIL is a really horrible thing.

-- 
Regards
Yafang





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