Re: [PATCH v3 4/4] mm: prohibit NULL deference exposed for unsupported non-blockable __GFP_NOFAIL

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On Mon, Aug 19, 2024 at 6:10 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Mon, Aug 19, 2024 at 9:46 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
> >
> > On Mon, Aug 19, 2024 at 5:39 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
> > >
> > > On Mon, Aug 19, 2024 at 9:25 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
> > > >
> > > > On Mon, Aug 19, 2024 at 3:50 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > > >
> > > > > On Sun 18-08-24 10:55:09, Yafang Shao wrote:
> > > > > > On Sat, Aug 17, 2024 at 2:25 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
> > > > > > >
> > > > > > > From: Barry Song <v-songbaohua@xxxxxxxx>
> > > > > > >
> > > > > > > When users allocate memory with the __GFP_NOFAIL flag, they might
> > > > > > > incorrectly use it alongside GFP_ATOMIC, GFP_NOWAIT, etc.  This kind of
> > > > > > > non-blockable __GFP_NOFAIL is not supported and is pointless.  If we
> > > > > > > attempt and still fail to allocate memory for these users, we have two
> > > > > > > choices:
> > > > > > >
> > > > > > >     1. We could busy-loop and hope that some other direct reclamation or
> > > > > > >     kswapd rescues the current process. However, this is unreliable
> > > > > > >     and could ultimately lead to hard or soft lockups,
> > > > > >
> > > > > > That can occur even if we set both __GFP_NOFAIL and
> > > > > > __GFP_DIRECT_RECLAIM, right?
> > > > >
> > > > > No, it cannot! With __GFP_DIRECT_RECLAIM the allocator might take a long
> > > > > time to satisfy the allocation but it will reclaim to get the memory, it
> > > > > will sleep if necessary and it will will trigger OOM killer if there is
> > > > > no other option. __GFP_DIRECT_RECLAIM is a completely different story
> > > > > than without it which means _no_sleeping_ is allowed and therefore only
> > > > > a busy loop waiting for the allocation to proceed is allowed.
> > > >
> > > > That could be a livelock.
> > > > From the user's perspective, there's no noticeable difference between
> > > > a livelock, soft lockup, or hard lockup.
> > >
> > > This is certainly different. A lockup occurs when tasks can't be scheduled,
> > > causing the entire system to stop functioning.
> >
> > When a livelock occurs, your only options are to migrate your
> > applications to other servers or reboot the system—there’s no other
> > resolution (except for using oomd, which is difficult for users
> > without cgroup2 or swap).
> >
> > So, there's effectively no difference.
>
> Could you express your options more clearly? I am guessing two
> possibilities?
> 1. entirely drop __GFP_NOFAIL and require all users who are
> using __GFP_NOFAIL to add error handlers instead?

When the system is unstable—such as after reaching the maximum retries
without successfully allocating pages—simply failing the operation
might be the better option.

>
> 2. no matter if it is an unsupported case, such as, GFP_ATOMIC|
> __GFP_NOFAIL, we always loop till a soft or hard lockup?

--
Regards
Yafang





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