Messages in this thread | | | Date | Thu, 17 Nov 2022 09:54:06 +0530 | Subject | Re: [PATCH 2/2] arm64/mm: fix incorrect file_map_count for invalid pmd/pud | From | Anshuman Khandual <> |
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On 11/16/22 21:16, Mark Rutland wrote: > On Wed, Nov 16, 2022 at 10:08:27AM +0100, David Hildenbrand wrote: >> On 16.11.22 09:38, Liu Shixin wrote: >>> The page table check trigger BUG_ON() unexpectedly when split hugepage: >>> >>> ------------[ cut here ]------------ >>> kernel BUG at mm/page_table_check.c:119! >>> Internal error: Oops - BUG: 00000000f2000800 [#1] SMP >>> Dumping ftrace buffer: >>> (ftrace buffer empty) >>> Modules linked in: >>> CPU: 7 PID: 210 Comm: transhuge-stres Not tainted 6.1.0-rc3+ #748 >>> Hardware name: linux,dummy-virt (DT) >>> pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--) >>> pc : page_table_check_set.isra.0+0x398/0x468 >>> lr : page_table_check_set.isra.0+0x1c0/0x468 >>> [...] >>> Call trace: >>> page_table_check_set.isra.0+0x398/0x468 >>> __page_table_check_pte_set+0x160/0x1c0 >>> __split_huge_pmd_locked+0x900/0x1648 >>> __split_huge_pmd+0x28c/0x3b8 >>> unmap_page_range+0x428/0x858 >>> unmap_single_vma+0xf4/0x1c8 >>> zap_page_range+0x2b0/0x410 >>> madvise_vma_behavior+0xc44/0xe78 >>> do_madvise+0x280/0x698 >>> __arm64_sys_madvise+0x90/0xe8 >>> invoke_syscall.constprop.0+0xdc/0x1d8 >>> do_el0_svc+0xf4/0x3f8 >>> el0_svc+0x58/0x120 >>> el0t_64_sync_handler+0xb8/0xc0 >>> el0t_64_sync+0x19c/0x1a0 >>> [...] >>> >>> On arm64, pmd_present() will return true even if the pmd is invalid. >> >> I assume that's because of the pmd_present_invalid() check. >> >> ... I wonder why that behavior was chosen. Sounds error-prone to me. > > That seems to be down to commit: > > b65399f6111b03df ("arm64/mm: Change THP helpers to comply with generic MM semantics") > > ... apparently because Andrea Arcangelli said this was necessary in: > > https://lore.kernel.org/lkml/20181017020930.GN30832@redhat.com/ > > ... but that does see to contradict what's said in: > > Documentation/mm/arch_pgtable_helpers.rst > > ... which just says: > > pmd_present Tests a valid mapped PMD
It should be as follows instead, will update. Not sure about PUD level though, where anon THP is not supported (AFAIK).
+---------------------------+--------------------------------------------------+ | pmd_present | Tests if pmd_page() points to valid memory page | +---------------------------+--------------------------------------------------+
> > ... and it's not clear to me why this *only* applies to the PMD level. > > Anshuman?
Because THP is supported at PMD level. As Andrea had explained earlier, pmd_present() should return positive if pmd_page() on the entry points to valid memory irrespective of whether the entry is valid/mapped or not. That is the semantics expected in generic THP during PMD split, collapse, migration etc and other memory code walking past such PMD entries. That was my understanding.
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