Re: [PATCH v1 05/11] KVM: s390: pv: refactor s390_reset_acc

From: Janosch Frank
Date: Wed May 26 2021 - 08:11:38 EST

On 5/17/21 10:07 PM, Claudio Imbrenda wrote:
> Refactor s390_reset_acc so that its pieces can be reused in upcoming
> patches. The users parameter for s390_destroy_range will be needed in
> upcoming patches.
>
> We don't want to hold all the locks used in a walk_page_range for too
> long, and the destroy page UVC does take some time to complete.
> Therefore we quickly gather the pages to destroy, and then destroy them
> without holding all the locks.
>

Acked-by: Janosch Frank <frankja@xxxxxxxxxxxxx>

> Signed-off-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
> ---
> arch/s390/include/asm/gmap.h | 5 +-
> arch/s390/kvm/pv.c | 12 ++++-
> arch/s390/mm/gmap.c | 88 ++++++++++++++++++++++++------------
> 3 files changed, 73 insertions(+), 32 deletions(-)
>
> diff --git a/arch/s390/include/asm/gmap.h b/arch/s390/include/asm/gmap.h
> index 40264f60b0da..618ddc455867 100644
> --- a/arch/s390/include/asm/gmap.h
> +++ b/arch/s390/include/asm/gmap.h
> @@ -147,5 +147,8 @@ int gmap_mprotect_notify(struct gmap *, unsigned long start,
> void gmap_sync_dirty_log_pmd(struct gmap *gmap, unsigned long dirty_bitmap[4],
> unsigned long gaddr, unsigned long vmaddr);
> int gmap_mark_unmergeable(void);
> -void s390_reset_acc(struct mm_struct *mm);
> +void s390_uv_destroy_range(struct mm_struct *mm, unsigned int users,
> + unsigned long start, unsigned long end);
> +void s390_uv_destroy_pfns(unsigned long count, unsigned long *pfns);
> +
> #endif /* _ASM_S390_GMAP_H */
> diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
> index e0532ab725bf..c3f9f30d2ed4 100644
> --- a/arch/s390/kvm/pv.c
> +++ b/arch/s390/kvm/pv.c
> @@ -12,6 +12,8 @@
> #include <asm/gmap.h>
> #include <asm/uv.h>
> #include <asm/mman.h>
> +#include <linux/pagewalk.h>
> +#include <linux/sched/mm.h>
> #include "kvm-s390.h"
>
> int kvm_s390_pv_destroy_cpu(struct kvm_vcpu *vcpu, u16 *rc, u16 *rrc)
> @@ -204,8 +206,14 @@ int kvm_s390_pv_deinit_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
> {
> int cc;
>
> - /* make all pages accessible before destroying the guest */
> - s390_reset_acc(kvm->mm);
> + /*
> + * if the mm still has a mapping, make all its pages accessible
> + * before destroying the guest
> + */
> + if (mmget_not_zero(kvm->mm)) {
> + s390_uv_destroy_range(kvm->mm, 0, 0, TASK_SIZE);
> + mmput(kvm->mm);
> + }
>
> cc = uv_cmd_nodata(kvm_s390_pv_get_handle(kvm),
> UVC_CMD_DESTROY_SEC_CONF, rc, rrc);
> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> index de679facc720..ad210a6e2c41 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -2670,41 +2670,71 @@ void s390_reset_cmma(struct mm_struct *mm)
> }
> EXPORT_SYMBOL_GPL(s390_reset_cmma);
>
> -/*
> - * make inaccessible pages accessible again
> - */
> -static int __s390_reset_acc(pte_t *ptep, unsigned long addr,
> - unsigned long next, struct mm_walk *walk)
> +#define DESTROY_LOOP_THRESHOLD 32
> +
> +struct reset_walk_state {
> + unsigned long next;
> + unsigned long count;
> + unsigned long pfns[DESTROY_LOOP_THRESHOLD];

Candidate for a module parameter and extensive performance testing?

> +};
> +
> +static int s390_gather_pages(pte_t *ptep, unsigned long addr,
> + unsigned long next, struct mm_walk *walk)

A "pv" somewhere in that function name would be helpful to me.
Also the "__" prefix applies here I think. We never call the function
directly and it's static. And I assume that's what the "__" prefix is
trying to tell us. :)

> {
> + struct reset_walk_state *p = walk->private;
> pte_t pte = READ_ONCE(*ptep);
>
> - /* There is a reference through the mapping */
> - if (pte_present(pte))
> - WARN_ON_ONCE(uv_destroy_owned_page(pte_val(pte) & PAGE_MASK));
> -
> - return 0;
> + if (pte_present(pte)) {
> + /* we have a reference from the mapping, take an extra one */
> + get_page(phys_to_page(pte_val(pte)));
> + p->pfns[p->count] = phys_to_pfn(pte_val(pte));
> + p->next = next;
> + p->count++;
> + }
> + return p->count >= DESTROY_LOOP_THRESHOLD;
> }
>
> -static const struct mm_walk_ops reset_acc_walk_ops = {
> - .pte_entry = __s390_reset_acc,
> +static const struct mm_walk_ops gather_pages_ops = {
> + .pte_entry = s390_gather_pages,
> };
>
> -#include <linux/sched/mm.h>
> -void s390_reset_acc(struct mm_struct *mm)
> +/*
> + * Call the Destroy secure page UVC on each page in the given array of PFNs.
> + * Each page needs to have an extra reference, which will be released here.
> + */
> +void s390_uv_destroy_pfns(unsigned long count, unsigned long *pfns)
> {
> - if (!mm_is_protected(mm))
> - return;
> - /*
> - * we might be called during
> - * reset: we walk the pages and clear
> - * close of all kvm file descriptors: we walk the pages and clear
> - * exit of process on fd closure: vma already gone, do nothing
> - */
> - if (!mmget_not_zero(mm))
> - return;
> - mmap_read_lock(mm);
> - walk_page_range(mm, 0, TASK_SIZE, &reset_acc_walk_ops, NULL);
> - mmap_read_unlock(mm);
> - mmput(mm);
> + unsigned long i;
> +
> + for (i = 0; i < count; i++) {
> + /* we always have an extra reference */
> + uv_destroy_owned_page(pfn_to_phys(pfns[i]));
> + /* get rid of the extra reference */
> + put_page(pfn_to_page(pfns[i]));
> + cond_resched();
> + }
> +}
> +EXPORT_SYMBOL_GPL(s390_uv_destroy_pfns);
> +
> +/*
> + * Walk the given range of the given address space, and call the destroy
> + * secure page UVC on each page.
> + * Exit early if the number of users of the mm drops to (or below) the given
> + * value.
> + */
> +void s390_uv_destroy_range(struct mm_struct *mm, unsigned int users,
> + unsigned long start, unsigned long end)
> +{
> + struct reset_walk_state state = { .next = start };
> + int r = 1;
> +
> + while ((r > 0) && (atomic_read(&mm->mm_users) > users)) {
> + state.count = 0;
> + mmap_read_lock(mm);
> + r = walk_page_range(mm, state.next, end, &gather_pages_ops, &state);
> + mmap_read_unlock(mm);
> + cond_resched();
> + s390_uv_destroy_pfns(state.count, state.pfns);
> + }
> }
> -EXPORT_SYMBOL_GPL(s390_reset_acc);
> +EXPORT_SYMBOL_GPL(s390_uv_destroy_range);
>