Messages in this thread | | | Subject | Re: linux-next: build failure on powerpc 8xx with 16k pages | From | Christophe Leroy <> | Date | Thu, 4 Jun 2020 13:55:45 +0000 |
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On 06/04/2020 11:17 AM, Will Deacon wrote: > Hi, [+Peter] > > On Thu, Jun 04, 2020 at 10:48:03AM +0000, Christophe Leroy wrote: >> Using mpc885_ads_defconfig with CONFIG_PPC_16K_PAGES instead of >> CONFIG_PPC_4K_PAGES, getting the following build failure: >> >> CC mm/gup.o >> In file included from ./include/linux/kernel.h:11:0, >> from mm/gup.c:2: >> In function 'gup_hugepte.constprop', >> inlined from 'gup_huge_pd.isra.78' at mm/gup.c:2465:8: >> ./include/linux/compiler.h:392:38: error: call to '__compiletime_assert_257' >> declared with attribute error: Unsupported access size for >> {READ,WRITE}_ONCE(). >> _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__) >> ^ >> ./include/linux/compiler.h:373:4: note: in definition of macro >> '__compiletime_assert' >> prefix ## suffix(); \ >> ^ >> ./include/linux/compiler.h:392:2: note: in expansion of macro >> '_compiletime_assert' >> _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__) >> ^ >> ./include/linux/compiler.h:405:2: note: in expansion of macro >> 'compiletime_assert' >> compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long), \ >> ^ >> ./include/linux/compiler.h:291:2: note: in expansion of macro >> 'compiletime_assert_rwonce_type' >> compiletime_assert_rwonce_type(x); \ >> ^ >> mm/gup.c:2428:8: note: in expansion of macro 'READ_ONCE' >> pte = READ_ONCE(*ptep); >> ^ >> In function 'gup_get_pte', >> inlined from 'gup_pte_range' at mm/gup.c:2228:9, >> inlined from 'gup_pmd_range' at mm/gup.c:2613:15, >> inlined from 'gup_pud_range' at mm/gup.c:2641:15, >> inlined from 'gup_p4d_range' at mm/gup.c:2666:15, >> inlined from 'gup_pgd_range' at mm/gup.c:2694:15, >> inlined from 'internal_get_user_pages_fast' at mm/gup.c:2785:3: > > At first glance, this looks like a real bug in the 16k page code -- you're > loading the pte non-atomically on the fast GUP path and so you're prone to > tearing, which probably isn't what you want. For a short-term hack, I'd > suggest having CONFIG_HAVE_FAST_GUP depend on !CONFIG_PPC_16K_PAGES, but if > you want to support this them you'll need to rework your pte_t so that it > can be loaded atomically.
What do you mean by *rework* pte_t ? pte are 32 bits words in size and are spread every 4 words in memory. Therefore pte_t has to be 128 bits because unlike huge_pte handling which always use huge_pte_offset() in loops, many many places in the kernel do pte++, so we need the pte type to be the size of the interval from one pte to the next one.
Christophe
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