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No problem!! It's a valuable learning experience for me. >>> - if (PageKsm(page)) >>> + if (mapping & PAGE_MAPPING_KSM) >>> u |= 1 << KPF_KSM; >> This might need an #ifdef? >> Say mapping is movable and anon -- then (mapping & PAGE_MAPPING_KSM) is >> true. Before, we called PageKsm, which falls through to a PG_ksm check. >> If !CONFIG_KSM then that flag is always false. But now, we're liable to >> report KPF_KSM even if !CONFIG_KSM. > > I'm not sure where you see a PG_ksm check: > > static __always_inline bool folio_test_ksm(const struct folio *folio) > { > return ((unsigned long)folio->mapping & PAGE_MAPPING_FLAGS) == > PAGE_MAPPING_KSM; > } > > static __always_inline bool PageKsm(const struct page *page) > { > return folio_test_ksm(page_folio(page)); > } My bad. What I meant was, if CONFIG_KSM is undefined, then > #ifdef CONFIG_KSM > ... > static __always_inline bool PageKsm(struct page *page) > { > return folio_test_ksm(page_folio(page)); > } will fall through to > # else > TESTPAGEFLAG_FALSE(Ksm, ksm) > #endif And you're right -- there is no PG_ksm comparison -- but the autogenerated PageKsm will always return false: > #define TESTPAGEFLAG_FALSE(uname, lname) \ > ... > static inline int Page##uname(const struct page *page) > { > return 0; > } But given your comments below, I'm realizing this isn't as important as I thought it was. > There's no such thing as a movable anon page -- the two bits in the > bottom of the mapping pointer mean: > > 00 file (or NULL) > 01 anon > 10 movable > 11 KSM > > Perhaps it might be clearer to say that anon pages are inherently > movable; the movable type really means that the reset of the mapping > pointer refers to a movable_operations instead of a mapping or anon_vma. I see. I misunderstood how the flags are applied. I thought that 11 == (01 | 10) -- i.e. that KSM was an intersection of MOVABLE and ANON. But they're more like mutually-exclusive states. And I doubt that a page will end up in the KSM "state" if CONFIG_KSM is disabled. So we don't need to rely on PageKsm() for the CONFIG_KSM check. That said, won't if (mapping & PAGE_MAPPING_KSM) return true even if a mapping is ANON (01) or MOVABLE (10) but not KSM (11)? Shouldn't this at least be if (mapping & PAGE_MAPPING_KSM == PAGE_MAPPING_KSM) ? >>> /* >>> * compound pages: export both head/tail info >>> * they together define a compound page's start/end pos and order >>> */ >>> - if (PageHead(page)) >>> - u |= 1 << KPF_COMPOUND_HEAD; >>> - if (PageTail(page)) >>> + if (page == &folio->page) >>> + u |= kpf_copy_bit(k, KPF_COMPOUND_HEAD, PG_head); >>> + else >>> u |= 1 << KPF_COMPOUND_TAIL; >> This makes sense but it'd require changes to the documentation. >> I ran a python3 memhog to see if anonymous pages are currently reported >> as COMPOUND_HEAD or COMPOUND_TAIL and it seems to be a no on both. >> But with this, I think every pfn will have one of the two set. >> Unless you can have a page outside of a folio -- not sure. > > I see your confusion. We have three cases; head, tail and neither > (obviously a page is never both head & tail). If a page is neither, > it's order-0 and it is the only page in the folio. So we handle head > or neither in the first leg of the 'if' where we set KPF_COMPOUND_HEAD > if PG_head is set, and tail in the 'else' leg. Dumb mistake on my part. For some reason, I thought that every folio->page had its PG_head set. > It's not so much the performance as it is the atomicity. I'm doing my > best to get an atomic snapshot of the flags and report a consistent > state, even if it might be stale by the time the user sees it. I see. That makes sense. Cool! Thanks for bearing with me. Beyond the KSM stuff, my only hangup is that this patch doesn't account for the handful of remaining per-page flags (KPF_HWPOISON, KPF_ARCH_*). Should I take this diff, tack those on in a second commit, and then put up a v4? Forgive me, I'm very green to the kernel dev process...