Re: [PATCH v3 02/14] arm64: drop ranges in definition of ARCH_FORCE_MAX_ORDER

[Justin Forbes]

On Tue, Apr 4, 2023 at 2:22 AM Mike Rapoport <rppt@xxxxxxxxxx> wrote:
>
> On Wed, Mar 29, 2023 at 10:55:37AM -0500, Justin Forbes wrote:
> > On Sat, Mar 25, 2023 at 1:09 AM Mike Rapoport <rppt@xxxxxxxxxx> wrote:
> > >
> > > From: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx>
> > >
> > > It is not a good idea to change fundamental parameters of core memory
> > > management. Having predefined ranges suggests that the values within
> > > those ranges are sensible, but one has to *really* understand
> > > implications of changing MAX_ORDER before actually amending it and
> > > ranges don't help here.
> > >
> > > Drop ranges in definition of ARCH_FORCE_MAX_ORDER and make its prompt
> > > visible only if EXPERT=y
> >
> > I do not like suddenly hiding this behind EXPERT for a couple of
> > reasons.  Most importantly, it will silently change the config for
> > users building with an old kernel config.  If a user has for instance
> > "13" set and building with 4K pages, as is the current configuration
> > for Fedora and RHEL aarch64 builds, an oldconfig build will now set it
> > to 10 with no indication that it is doing so.  And while I think that
> > 10 is a fine default for many aarch64 users, there are valid reasons
> > for choosing other values. Putting this behind expert makes it much
> > less obvious that this is an option.
>
> That's the idea of EXPERT, no?
>
> This option was intended to allow allocation of huge pages for
> architectures that had PMD_ORDER > MAX_ORDER and not to allow user to
> select size of maximal physically contiguous allocation.
>
> Changes to MAX_ORDER fundamentally change the behaviour of core mm and
> unless users *really* know what they are doing there is no reason to choose
> non-default values so hiding this option behind EXPERT seems totally
> appropriate to me.

It sounds nice in theory. In practice. EXPERT hides too much. When you
flip expert, you expose over a 175ish new config options which are
hidden behind EXPERT.  You don't have to know what you are doing just
with the MAX_ORDER, but a whole bunch more as well.  If everyone were
already running 10, this might be less of a problem. At least Fedora
and RHEL are running 13 for 4K pages on aarch64. This was not some
accidental choice, we had to carry a patch to even allow it for a
while.  If this does go in as is, we will likely just carry a patch to
remove the "if EXPERT", but that is a bit of a disservice to users who
might be trying to debug something else upstream, bisecting upstream
kernels or testing a patch.  In those cases, people tend to use
pristine upstream sources without distro patches to verify, and they
tend to use their existing configs. With this change, their MAX_ORDER
will drop to 10 from 13 silently.   That can look like a different
issue enough to ruin a bisect or have them give bad feedback on a
patch because it introduces a "regression" which is not a regression
at all, but a config change they couldn't see.

>
> > Justin
> >
> > > Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> > > Signed-off-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
> > > ---
> > >  arch/arm64/Kconfig | 4 +---
> > >  1 file changed, 1 insertion(+), 3 deletions(-)
> > >
> > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > index e60baf7859d1..7324032af859 100644
> > > --- a/arch/arm64/Kconfig
> > > +++ b/arch/arm64/Kconfig
> > > @@ -1487,11 +1487,9 @@ config XEN
> > >  # 16K |       27          |      14      |       13        |         11         |
> > >  # 64K |       29          |      16      |       13        |         13         |
> > >  config ARCH_FORCE_MAX_ORDER
> > > -       int "Maximum zone order" if ARM64_4K_PAGES || ARM64_16K_PAGES
> > > +       int "Maximum zone order" if EXPERT && (ARM64_4K_PAGES || ARM64_16K_PAGES)
> > >         default "13" if ARM64_64K_PAGES
> > > -       range 11 13 if ARM64_16K_PAGES
> > >         default "11" if ARM64_16K_PAGES
> > > -       range 10 15 if ARM64_4K_PAGES
> > >         default "10"
> > >         help
> > >           The kernel memory allocator divides physically contiguous memory
> > > --
> > > 2.35.1
> > >
> > >
>
> --
> Sincerely yours,
> Mike.
>