On Tue, Apr 04, 2023 at 06:50:01AM -0500, Justin Forbes wrote: > 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. If we remove EXPERT (as prior to this patch), I'd rather keep the ranges and avoid having to explain to people why some random MAX_ORDER doesn't build (keeping the range would also make sense for randconfig, not sure we got to any conclusion there). -- Catalin
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