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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> 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.
