Messages in this thread | | | Date | Mon, 21 Nov 2022 12:27:51 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH] arm64/mm: Drop redundant BUG_ON(!pgtable_alloc) |
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On Mon, Nov 21, 2022 at 11:00:42AM +0530, Anshuman Khandual wrote: > Hello Nathan, > > Thanks for the report. > > On 11/20/22 21:46, Nathan Chancellor wrote: > > Hi Anshuman,
> > I just bisected a boot failure in our QEMU-based continuous integration > > setup to this change as commit 9ed2b4616d4e ("arm64/mm: Drop redundant > > BUG_ON(!pgtable_alloc)") in the arm64 tree. There is no output so the > > panic clearly happens early at boot. If I move back to the previous > > commit and add a WARN_ON() like so: > > > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > > index d386033a074c..9280a92ff920 100644 > > --- a/arch/arm64/mm/mmu.c > > +++ b/arch/arm64/mm/mmu.c > > @@ -383,6 +383,7 @@ static void __create_pgd_mapping_locked(pgd_t *pgdir, phys_addr_t phys, > > phys &= PAGE_MASK; > > addr = virt & PAGE_MASK; > > end = PAGE_ALIGN(virt + size); > > + WARN_ON(!pgtable_alloc); > > > > do { > > next = pgd_addr_end(addr, end); > > > > I do see some stacktraces. I have attached the boot log from QEMU. > > > > If there is any additional information I can provide or patches I can > > test, I am more than happy to do so. > > There are couple of instances, where __create_pgd_mapping() function gets called > without a valid pgtable alloc function (NULL is passed on instead), as it is not > expected to allocate page table pages, during the mapping process. The following > change after this patch should solve the reported problem. > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index 9ea8e9039992..a00563122fcb 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -42,6 +42,7 @@ > #define NO_BLOCK_MAPPINGS BIT(0) > #define NO_CONT_MAPPINGS BIT(1) > #define NO_EXEC_MAPPINGS BIT(2) /* assumes FEAT_HPDS is not used */ > +#define NO_ALLOC_MAPPINGS BIT(3) /* does not allocate page table pages */ > > int idmap_t0sz __ro_after_init; > > @@ -380,7 +381,7 @@ static void __create_pgd_mapping_locked(pgd_t *pgdir, phys_addr_t phys, > phys &= PAGE_MASK; > addr = virt & PAGE_MASK; > end = PAGE_ALIGN(virt + size); > - BUG_ON(!pgtable_alloc); > + BUG_ON(!(flags & NO_ALLOC_MAPPINGS) && !pgtable_alloc); > > do { > next = pgd_addr_end(addr, end); > @@ -453,7 +454,7 @@ static void __init create_mapping_noalloc(phys_addr_t phys, unsigned long virt, > return; > } > __create_pgd_mapping(init_mm.pgd, phys, virt, size, prot, NULL, > - NO_CONT_MAPPINGS); > + NO_CONT_MAPPINGS | NO_ALLOC_MAPPINGS); > } > > void __init create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys, > @@ -481,7 +482,7 @@ static void update_mapping_prot(phys_addr_t phys, unsigned long virt, > } > > __create_pgd_mapping(init_mm.pgd, phys, virt, size, prot, NULL, > - NO_CONT_MAPPINGS); > + NO_CONT_MAPPINGS | NO_ALLOC_MAPPINGS); > > /* flush the TLBs after updating live kernel mappings */ > flush_tlb_kernel_range(virt, virt + size);
This is now more complicated than what we had originally, and it doesn't catch the case where the caller sets NO_ALLOC_MAPPINGS but the callee ends up needing to perform an allocation, which the old code would have caught.
This is clearly more subtle than we thought initially; for now could we please just drop the patch?
Thanks, Mark.
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