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. >
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