Re: [RFC PATCH] mm/vmalloc: fix vmalloc which may return null if called with __GFP_NOFAIL

[Barry Song <21cnbao@gmail.com>]

On Thu, May 9, 2024 at 3:33 PM Hailong Liu <hailong.liu@oppo.com> wrote:
>
> On Thu, 09. May 14:20, Barry Song wrote:
> > On Thu, May 9, 2024 at 12:58 AM <hailong.liu@oppo.com> wrote:
> > >
> > > From: "Hailong.Liu" <hailong.liu@oppo.com>
> > >
> > > Commit a421ef303008 ("mm: allow !GFP_KERNEL allocations for kvmalloc")
> > > includes support for __GFP_NOFAIL, but it presents a conflict with
> > > commit dd544141b9eb ("vmalloc: back off when the current task is
> > > OOM-killed"). A possible scenario is as belows:
> > >
> > > process-a
> > > kvcalloc(n, m, GFP_KERNEL | __GFP_NOFAIL)
> > >     __vmalloc_node_range()
> > >         __vmalloc_area_node()
> > >             vm_area_alloc_pages()
> > >             --> oom-killer send SIGKILL to process-a
> > >             if (fatal_signal_pending(current)) break;
> > > --> return NULL;
> > >
> > > to fix this, do not check fatal_signal_pending() in vm_area_alloc_pages()
> > > if __GFP_NOFAIL set.
> > >
> > > Reported-by: Oven <liyangouwen1@oppo.com>
> > > Signed-off-by: Hailong.Liu <hailong.liu@oppo.com>
> > > ---
> > >  mm/vmalloc.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index 6641be0ca80b..2f359d08bf8d 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -3560,7 +3560,7 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
> > >
> > >         /* High-order pages or fallback path if "bulk" fails. */
> > >         while (nr_allocated < nr_pages) {
> > > -               if (fatal_signal_pending(current))
> > > +               if (!(gfp & __GFP_NOFAIL) && fatal_signal_pending(current))
> > >                         break;
> >
> > why not !nofail ?
>
> if order = 0, nofail would not be set true in bulk allocator. in such a case,
> it is still possible to break early
>
> >
> > This seems a correct fix, but it undermines the assumption made in
> > commit dd544141b9eb
> >  ("vmalloc: back off when the current task is OOM-killed")
> >
> > "
> >     This may trigger some hidden problems, when caller does not handle
> >     vmalloc failures, or when rollaback after failed vmalloc calls own
> >     vmallocs inside.  However all of these scenarios are incorrect: vmalloc
> >     does not guarantee successful allocation, it has never been called with
> >     __GFP_NOFAIL and threfore either should not be used for any rollbacks or
> >     should handle such errors correctly and not lead to critical failures.
> > "
> >
> > If a significant kvmalloc operation is performed with the NOFAIL flag, it risks
> > reverting the fix intended to address the OOM-killer issue in commit
> > dd544141b9eb.
> > Should we indeed permit the NOFAIL flag for large kvmalloc allocations?
>
> IMO, if we encounter this issue, it should be fixed by the
> caller, not here.

I agree. but could we WARN_ON a large kvmalloc(NOFAIL) allocation?

> >
>
> >
> > Thanks
> > Barry
>
> --
>
> Best Regards,
> Hailong.