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

[Hailong Liu <hailong.liu@oppo.com>]

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.
>

>
> Thanks
> Barry

--

Best Regards,
Hailong.