Messages in this thread | | | Date | Mon, 13 Dec 2021 13:45:03 +0000 | From | Will Deacon <> | Subject | Re: [PATCH v2] arm64/mm: avoid fixmap race condition when create pud mapping |
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On Mon, Dec 13, 2021 at 11:35:16AM +0100, Ard Biesheuvel wrote: > On Mon, 13 Dec 2021 at 11:16, Anshuman Khandual > <anshuman.khandual@arm.com> wrote: > > On 12/10/21 3:24 PM, Jianyong Wu wrote: > > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > > > index acfae9b41cc8..98ac09ae9588 100644 > > > --- a/arch/arm64/mm/mmu.c > > > +++ b/arch/arm64/mm/mmu.c > > > @@ -63,6 +63,7 @@ static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused; > > > static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused; > > > > > > static DEFINE_SPINLOCK(swapper_pgdir_lock); > > > +static DEFINE_SPINLOCK(fixmap_lock); > > > > > > void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd) > > > { > > > @@ -329,6 +330,11 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end, > > > } > > > BUG_ON(p4d_bad(p4d)); > > > > > > + /* > > > + * fixmap is global resource, thus it needs to be protected by a lock > > > + * in case of race condition. > > > + */ > > > > Small nit, format and align this comment block. I guess > > could also be done while merging this patch as well. > > > > > + spin_lock(&fixmap_lock); > > > pudp = pud_set_fixmap_offset(p4dp, addr); > > > do { > > > pud_t old_pud = READ_ONCE(*pudp); > > > @@ -359,6 +365,7 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end, > > > } while (pudp++, addr = next, addr != end); > > > > > > pud_clear_fixmap(); > > > + spin_unlock(&fixmap_lock); > > > } > > > > > > static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys, > > > > > > > Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com> > > We have different fixmap slots for different page table levels, so > 'fixmap_lock' is not the right name. > > But we already have swapper_pgdir_lock as well, which serializes the > use of the pgdir level fixmap slot. And we have no spinlocks > protecting the other levels. > > So should we perhaps clean this up more comprehensively? Wouldn't it > be better to add a mutex to __create_pgd_mapping(), for instance?
That does sound like a better way to do things, but the simplicity of this patch is quite attractive for backporting. Would you object to me queuing it as-is, on the premise that I'm more than happy to take consolidation changes on top?
Will
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