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2025 at 03:16:52PM +0100, Matthew Wilcox wrote: > On Fri, May 02, 2025 at 02:12:16PM +0100, Lorenzo Stoakes wrote: > > On Fri, May 02, 2025 at 02:03:48PM +0100, Matthew Wilcox wrote: > > > On Fri, May 02, 2025 at 11:31:41AM +0200, Ignacio Moreno Gonzalez via B4 Relay wrote: > > > > From: Ignacio Moreno Gonzalez > > > > > > > > commit c4608d1bf7c6 ("mm: mmap: map MAP_STACK to VM_NOHUGEPAGE") maps > > > > the mmap option MAP_STACK to VM_NOHUGEPAGE. This is also done if > > > > CONFIG_TRANSPARENT_HUGETABLES is not defined. But in that case, the > > > > VM_NOHUGEPAGE does not make sense. For instance, when calling madvise() > > > > with MADV_NOHUGEPAGE, an error is always returned. > > > > > > Isn't that the real problem though? > > > > Hmm, but wouldn't we want users who are trying to set MADV_[NO]HUGEPAGE to > > be made aware that it isn't going to do anything? > > ... I thought the patch was clear. Only setting NOHUGEPAGE becomes a > no-op. Setting HUGEPAGE remains EINVAL. Sorry I"m an idiot, glossed over the 'if (advice == MADV_NOHUGEPAGE)' bit... > > > And wouldn't changing this be a possibly 'breaking userspace' thing if > > somebody somewhere relies on this? > > I don't see what userspace could rely on it returning EINVAL, since it > won't on a kernel which has THP enabled. I mean for the purposes of detecting THP being disabled, but I mean it's far-fetched. > > > Also this will make this inconsistent with e.g. MADV_COLLAPSE also? > > Not sure how ... > Yup this was based on my misreading... But I'm still not a fan of us setting the VM_NOHUGEPAGE flag like this when it's meaningless and this is (probably) the only way you can do it on a non-THP kernel. It's user-visible in /proc/$pid/smaps (I mean, that's the whole problem here right?) and impacts merging (though MAP_STACK you'd think it'd probably not be a big deal for...) Or ifdef the VM_[NO]HUGEPAGE on CONFIG_TRANSPARENT_HUGEPAGE? So maybe, let's do both things?