Re: [PATCH] vduse: avoid using __GFP_NOFAIL

[Jason Wang]

On Tue, Aug 6, 2024 at 10:30 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Mon, Aug 5, 2024 at 8:21 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> >
> > Barry said [1]:
> >
> > """
> > mm doesn't support non-blockable __GFP_NOFAIL allocation. Because
> > __GFP_NOFAIL without direct reclamation may just result in a busy
> > loop within non-sleepable contexts.
> > ""“
>
> the current code will result in returning a NULL pointer but
> not a busy-loop.
>
> static inline struct page *
> __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>                                                 struct alloc_context *ac)
> {
>         ...
>         /*
>          * Make sure that __GFP_NOFAIL request doesn't leak out and make sure
>          * we always retry
>          */
>         if (gfp_mask & __GFP_NOFAIL) {
>                 /*
>                  * All existing users of the __GFP_NOFAIL are blockable, so warn
>                  * of any new users that actually require GFP_NOWAIT
>                  */
>                 if (WARN_ON_ONCE_GFP(!can_direct_reclaim, gfp_mask))
>                         goto fail;
>                 ...
>         }
>         ...
> fail:
>         warn_alloc(gfp_mask, ac->nodemask,
>                         "page allocation failure: order:%u", order);
> got_pg:
>         return page;
> }
>
>
> We have two choices to address the issue:
> 1. busy-loop
> 2. BUG_ON
>
> the below patch chose 2:
> https://lore.kernel.org/linux-mm/20240731000155.109583-5-21cnbao@xxxxxxxxx/
>

I will add those to V2.

Thanks