Re: [PATCH V2] x86/mm: Split vmalloc_sync_all()

Rafael J. Wysocki
2019-10-11T09:35:54Z

On Wednesday, October 9, 2019 2:44:18 PM CEST Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> Git commit 3f8fd02b1bf1 ("mm/vmalloc: Sync unmappings in
> __purge_vmap_area_lazy()") introduced a call to vmalloc_sync_all() in
> the vunmap() code-path. While this change was necessary to maintain
> correctness on x86-32-pae kernels, it also adds additional cycles for
> architectures that don't need it.
>
> Specifically on x86-64 with CONFIG_VMAP_STACK=y some people reported
> severe performance regressions in micro-benchmarks because it now also
> calls the x86-64 implementation of vmalloc_sync_all() on vunmap(). But
> the vmalloc_sync_all() implementation on x86-64 is only needed for
> newly created mappings.
>
> To avoid the unnecessary work on x86-64 and to gain the performance
> back, split up vmalloc_sync_all() into two functions:
>
> * vmalloc_sync_mappings(), and
> * vmalloc_sync_unmappings()
>
> Most call-sites to vmalloc_sync_all() only care about new mappings
> being synchronized. The only exception is the new call-site
> added in the above mentioned commit.
>
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Fixes: 3f8fd02b1bf1 ("mm/vmalloc: Sync unmappings in __purge_vmap_area_lazy()")
> Signed-off-by: Joerg Roedel <jroedel@suse.de>

For the GHES bits

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

> ---
> Changes to initial post:
> - Added comments to x86-64 version of
> vmalloc_sync_[un]mappings() as suggested by Dave
> Hansen.
>
> arch/x86/mm/fault.c | 26 ++++++++++++++++++++++++--
> drivers/acpi/apei/ghes.c | 2 +-
> include/linux/vmalloc.h | 5 +++--
> kernel/notifier.c | 2 +-
> mm/nommu.c | 10 +++++++---
> mm/vmalloc.c | 11 +++++++----
> 6 files changed, 43 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 9ceacd1156db..94174361f524 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -189,7 +189,7 @@ static inline pmd_t *vmalloc_sync_one(pgd_t *pgd, unsigned long address)
> return pmd_k;
> }
>
> -void vmalloc_sync_all(void)
> +static void vmalloc_sync(void)
> {
> unsigned long address;
>
> @@ -216,6 +216,16 @@ void vmalloc_sync_all(void)
> }
> }
>
> +void vmalloc_sync_mappings(void)
> +{
> + vmalloc_sync();
> +}
> +
> +void vmalloc_sync_unmappings(void)
> +{
> + vmalloc_sync();
> +}
> +
> /*
> * 32-bit:
> *
> @@ -318,11 +328,23 @@ static void dump_pagetable(unsigned long address)
>
> #else /* CONFIG_X86_64: */
>
> -void vmalloc_sync_all(void)
> +void vmalloc_sync_mappings(void)
> {
> + /*
> + * 64-bit mappings might allocate new p4d/pud pages
> + * that need to be propagated to all tasks' PGDs.
> + */
> sync_global_pgds(VMALLOC_START & PGDIR_MASK, VMALLOC_END);
> }
>
> +void vmalloc_sync_unmappings(void)
> +{
> + /*
> + * Unmappings never allocate or free p4d/pud pages.
> + * No work is required here.
> + */
> +}
> +
> /*
> * 64-bit:
> *
> diff --git a/drivers/acpi/apei/ghes.c b/drivers/acpi/apei/ghes.c
> index 777f6f7122b4..e0d82fab1f44 100644
> --- a/drivers/acpi/apei/ghes.c
> +++ b/drivers/acpi/apei/ghes.c
> @@ -171,7 +171,7 @@ int ghes_estatus_pool_init(int num_ghes)
> * New allocation must be visible in all pgd before it can be found by
> * an NMI allocating from the pool.
> */
> - vmalloc_sync_all();
> + vmalloc_sync_mappings();
>
> rc = gen_pool_add(ghes_estatus_pool, addr, PAGE_ALIGN(len), -1);
> if (rc)
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 4e7809408073..decac0790fc1 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -126,8 +126,9 @@ extern int remap_vmalloc_range_partial(struct vm_area_struct *vma,
>
> extern int remap_vmalloc_range(struct vm_area_struct *vma, void *addr,
> unsigned long pgoff);
> -void vmalloc_sync_all(void);
> -
> +void vmalloc_sync_mappings(void);
> +void vmalloc_sync_unmappings(void);
> +
> /*
> * Lowlevel-APIs (not for driver use!)
> */
> diff --git a/kernel/notifier.c b/kernel/notifier.c
> index d9f5081d578d..157d7c29f720 100644
> --- a/kernel/notifier.c
> +++ b/kernel/notifier.c
> @@ -554,7 +554,7 @@ NOKPROBE_SYMBOL(notify_die);
>
> int register_die_notifier(struct notifier_block *nb)
> {
> - vmalloc_sync_all();
> + vmalloc_sync_mappings();
> return atomic_notifier_chain_register(&die_chain, nb);
> }
> EXPORT_SYMBOL_GPL(register_die_notifier);
> diff --git a/mm/nommu.c b/mm/nommu.c
> index 99b7ec318824..3b67bd20c2af 100644
> --- a/mm/nommu.c
> +++ b/mm/nommu.c
> @@ -359,10 +359,14 @@ void vm_unmap_aliases(void)
> EXPORT_SYMBOL_GPL(vm_unmap_aliases);
>
> /*
> - * Implement a stub for vmalloc_sync_all() if the architecture chose not to
> - * have one.
> + * Implement a stub for vmalloc_sync_[un]mapping() if the architecture
> + * chose not to have one.
> */
> -void __weak vmalloc_sync_all(void)
> +void __weak vmalloc_sync_mappings(void)
> +{
> +}
> +
> +void __weak vmalloc_sync_unmappings(void)
> {
> }
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index a3c70e275f4e..c0be707db434 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1259,7 +1259,7 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
> * First make sure the mappings are removed from all page-tables
> * before they are freed.
> */
> - vmalloc_sync_all();
> + vmalloc_sync_unmappings();
>
> /*
> * TODO: to calculate a flush range without looping.
> @@ -3050,16 +3050,19 @@ int remap_vmalloc_range(struct vm_area_struct *vma, void *addr,
> EXPORT_SYMBOL(remap_vmalloc_range);
>
> /*
> - * Implement a stub for vmalloc_sync_all() if the architecture chose not to
> - * have one.
> + * Implement stubs for vmalloc_sync_[un]mappings () if the architecture chose
> + * not to have one.
> *
> * The purpose of this function is to make sure the vmalloc area
> * mappings are identical in all page-tables in the system.
> */
> -void __weak vmalloc_sync_all(void)
> +void __weak vmalloc_sync_mappings(void)
> {
> }
>
> +void __weak vmalloc_sync_unmappings(void)
> +{
> +}
>
> static int f(pte_t *pte, unsigned long addr, void *data)
> {
>