Messages in this thread | | | Date | Wed, 11 Nov 2020 16:52:51 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH V9 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE |
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On Wed, Nov 11, 2020 at 03:30:22PM +0000, Matthew Wilcox wrote: > On Wed, Nov 11, 2020 at 01:43:57PM +0100, Peter Zijlstra wrote: > > + if (pud_leaf(pud)) { > > #ifdef pud_page > > - page = pud_page(*pud); > > - if (PageHuge(page)) > > - return page_size(compound_head(page)); > > + if (!pud_devmap(pud)) { > > + page = pud_page(pud); > > + if (PageHuge(page)) > > + return page_size(compound_head(page)); > > + } > > #endif > > return 1ULL << PUD_SHIFT; > > This confuses me. Why only special-case hugetlbfs pages here? Should > they really be treated differently from THP?
Do we have non-pagetable aligned THP ? I thought THP was always PUD sized.
> If you want to consider that we might be mapping a page that's twice > as big as a PUD entry and this is only half of it, then the simple way > is: > > if (pud_leaf(pud)) { > #ifdef pud_page > page = compound_head(pud_page(*pud)); > return page_size(page); > #else > return 1ULL << PUD_SHIFT; > #endif > } > > Also, what's up with the special-casing of devmap pages here? Did the > devmap people fuck up their compound pages? If so, they should fix their > shit, not expect the rest of the kernel to work around this brokenness.
Well, the PTE code we have today (in tip/perf/core) is:
pte = pte_offset_map(pmd, addr); if (!pte_present(*pte)) { pte_unmap(pte); return 0; }
page = pte_page(*pte); if (PageHuge(page)) { u64 size = page_size(compound_head(page)); pte_unmap(pte); return size; }
pte_unmap(pte); return PAGE_SIZE;
and that's crashing in PageHuge()'s PageCompound() test. Clearly I should be checking pte_special() here (as well as all the READ_ONCE()s I added in the patch you just commented on). But I wasn't quite sure about devmap and paranoia won.
You're saying devmap should be valid compound pages? Then I can remove all that and only keep pte_special().
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