Re: [PATCH v10 08/14] mm: multi-gen LRU: support page table walks

[Yu Zhao <yuzhao@google.com>]

On Fri, Apr 15, 2022 at 1:15 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Fri, 15 Apr 2022 00:25:45 -0600 Yu Zhao <yuzhao@google.com> wrote:
>
> > On Thu, Apr 14, 2022 at 7:57 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > >
> > > On Thu, 14 Apr 2022 19:14:54 -0600 Yu Zhao <yuzhao@google.com> wrote:
> > >
> > > > On Mon, Apr 11, 2022 at 8:16 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > > > >
> > > > > On Wed,  6 Apr 2022 21:15:20 -0600 Yu Zhao <yuzhao@google.com> wrote:
> > > > >
> > > > > > +static void update_batch_size(struct lru_gen_mm_walk *walk, struct folio *folio,
> > > > > > +                           int old_gen, int new_gen)
> > > > > > +{
> > > > > > +     int type = folio_is_file_lru(folio);
> > > > > > +     int zone = folio_zonenum(folio);
> > > > > > +     int delta = folio_nr_pages(folio);
> > > > > > +
> > > > > > +     VM_BUG_ON(old_gen >= MAX_NR_GENS);
> > > > > > +     VM_BUG_ON(new_gen >= MAX_NR_GENS);
> > > > >
> > > > > General rule: don't add new BUG_ONs, because they crash the kenrel.
> > > > > It's better to use WARN_ON or WARN_ON_ONCE then try to figure out a way
> > > > > to keep the kernel limping along.  At least so the poor user can gather logs.
> > > >
> > > > These are VM_BUG_ONs, which are BUILD_BUG_ONs except for (mostly MM) developers.
> > >
> > > I'm told that many production builds enable runtime VM_BUG_ONning.
> >
> > Nobody wants to debug VM in production. Some distros that offer both
> > the latest/LTS kernels do enable CONFIG_DEBUG_VM in the former so the
> > latter can have better test coverage when it becomes available. Do
> > people use the former in production? Absolutely, otherwise we won't
> > have enough test coverage. Are we supposed to avoid CONFIG_DEBUG_VM? I
> > don't think so, because it defeats the purpose of those distros
> > enabling it in the first place.
> >
> > The bottomline is that none of RHEL 8.5, SLES 15, Debian 11 enables
> > CONFIG_DEBUG_VM.
>
> I grabbed
> https://kojipkgs.fedoraproject.org//packages/kernel/5.18.0/0.rc2.23.fc37/src/kernel-5.18.0-0.rc2.23.fc37.src.rpm
> and

Yes, Fedora/RHEL is one concrete example of the model I mentioned
above (experimental/stable). I added Justin, the Fedora kernel
maintainer, and he can further clarify.

If we don't want more VM_BUG_ONs, I'll remove them. But (let me
reiterate) it seems to me that just defeats the purpose of having
CONFIG_DEBUG_VM.

> hp2:/home/akpm/yy> grep "DEBUG_VM[= ]" *.config
> kernel-aarch64-debug-fedora.config:CONFIG_DEBUG_VM=y
> kernel-aarch64-debug-rhel.config:# CONFIG_DEBUG_VM is not set
> kernel-aarch64-fedora.config:CONFIG_DEBUG_VM=y
> kernel-aarch64-rhel.config:# CONFIG_DEBUG_VM is not set
> kernel-armv7hl-debug-fedora.config:CONFIG_DEBUG_VM=y
> kernel-armv7hl-fedora.config:CONFIG_DEBUG_VM=y
> kernel-armv7hl-lpae-debug-fedora.config:CONFIG_DEBUG_VM=y
> kernel-armv7hl-lpae-fedora.config:CONFIG_DEBUG_VM=y
> kernel-ppc64le-debug-fedora.config:CONFIG_DEBUG_VM=y
> kernel-ppc64le-debug-rhel.config:CONFIG_DEBUG_VM=y
> kernel-ppc64le-fedora.config:CONFIG_DEBUG_VM=y
> kernel-ppc64le-rhel.config:# CONFIG_DEBUG_VM is not set
> kernel-s390x-debug-fedora.config:CONFIG_DEBUG_VM=y
> kernel-s390x-debug-rhel.config:CONFIG_DEBUG_VM=y
> kernel-s390x-fedora.config:CONFIG_DEBUG_VM=y
> kernel-s390x-rhel.config:# CONFIG_DEBUG_VM is not set
> kernel-s390x-zfcpdump-rhel.config:# CONFIG_DEBUG_VM is not set
> kernel-x86_64-debug-fedora.config:CONFIG_DEBUG_VM=y
> kernel-x86_64-debug-rhel.config:CONFIG_DEBUG_VM=y
> kernel-x86_64-fedora.config:CONFIG_DEBUG_VM=y
> kernel-x86_64-rhel.config:# CONFIG_DEBUG_VM is not set