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

[Mike Rapoport]

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.
 
> 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.