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Date:   Wed, 8 Jun 2022 04:25:24 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Dan Carpenter <dan.carpenter@oracle.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        syzbot <syzbot+dd3c97de244683533381@syzkaller.appspotmail.com>,
        hdanton@sina.com, lenb@kernel.org, linux-acpi@vger.kernel.org,
        linux-kernel@vger.kernel.org, rafael.j.wysocki@intel.com,
        rafael@kernel.org, rjw@rjwysocki.net,
        syzkaller-bugs@googlegroups.com, linux-usb@vger.kernel.org,
        Linux-MM <linux-mm@kvack.org>
Subject: Re: [syzbot] general protection fault in __device_attach
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On Tue, Jun 07, 2022 at 09:15:09AM +0200, Dmitry Vyukov wrote:
> On Mon, 6 Jun 2022 at 14:39, Dan Carpenter <dan.carpenter@oracle.com> wrote:
> >
> > On Sat, Jun 04, 2022 at 10:32:46AM +0200, 'Dmitry Vyukov' via syzkaller-bugs wrote:
> > > On Fri, 3 Jun 2022 at 18:12, Greg KH <gregkh@linuxfoundation.org> wrote:
> > > >
> > > > But again, is this a "real and able to be triggered from userspace"
> > > > problem, or just fault-injection-induced?
> > >
> > > Then this is something to fix in the fault injection subsystem.
> > > Testing systems shouldn't be reporting false positives.
> > > What allocations cannot fail in real life? Is it <=page_size?
> > >
> >
> > Apparently in 2014, anything less than *EIGHT?!!* pages succeeded!
> >
> > https://lwn.net/Articles/627419/
> >
> > I have been on the look out since that article and never seen anyone
> > mention it changing.  I think we should ignore that and say that
> > anything over PAGE_SIZE can fail.  Possibly we could go smaller than
> > PAGE_SIZE...
> 
> +linux-mm for GFP expertise re what allocations cannot possibly fail
> and should be excluded from fault injection.
> 
> Interesting, thanks for the link.
> 
> PAGE_SIZE looks like a good start. Once we have the predicate in
> place, we can refine it later when/if we have more inputs.
> 
> But I wonder about GFP flags. They definitely have some impact on allocations.
> If GFP_ACCOUNT is set, all allocations can fail, right?
> If GFP_DMA/DMA32 is set, allocations can fail, right? What about other zones?
> If GFP_NORETRY is set, allocations can fail?
> What about GFP_NOMEMALLOC and GFP_ATOMIC?
> What about GFP_IO/GFP_FS/GFP_DIRECT_RECLAIM/GFP_KSWAPD_RECLAIM? At
> least some of these need to be set for allocations to not fail? Which
> ones?
> Any other flags are required to be set/unset for allocations to not fail?

I'm not the expert on page allocation, but ...

I don't think GFP_ACCOUNT makes allocations fail.  It might make reclaim
happen from within that cgroup, and it might cause an OOM kill for
something in that cgroup.  But I don't think it makes a (low order)
allocation more likely to fail.

There's usually less memory avilable in DMA/DMA32 zones, but we have
so few allocations from those zones, I question the utility of focusing
testing on those allocations.

GFP_ATOMIC allows access to emergency pools, so I would say _less_ likely
to fail.  KSWAPD_RECLAIM has no effect on whether _this_ allocation
succeeds or fails; it kicks kswapd to do reclaim, rather than doing
reclaim directly.  DIRECT_RECLAIM definitely makes allocations more likely
to succeed.  GFP_FS allows (direct) reclaim to happen from filesystems.
GFP_IO allows IO to start (ie writeback can start) in order to clean
dirty memory.

Anyway, I hope somebody who knows the page allocator better than I do
can say smarter things than this.  Even better if they can put it into
Documentation/ somewhere ;-)

https://www.kernel.org/doc/html/latest/core-api/memory-allocation.html
exists but isn't quite enough to answer this question.