Re: [PATCH v7 04/17] KVM: s390: pv: refactor s390_reset_acc

Janosch Frank
2022-02-07T10:23:20Z

On 2/4/22 16:53, Claudio Imbrenda wrote:
> Refactor s390_reset_acc so that it can be reused 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.
>
> The new refactored function optionally allows to return early without
> completing if a fatal signal is pending (and return and appropriate
> error code). Two wrappers are provided to call the new function.
>
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> (dropping Janosch's Ack because of major changes to the patch)
> ---
> arch/s390/include/asm/gmap.h | 36 +++++++++++++-
> arch/s390/kvm/pv.c | 12 ++++-
> arch/s390/mm/gmap.c | 95 +++++++++++++++++++++++++-----------
> 3 files changed, 111 insertions(+), 32 deletions(-)
>
> diff --git a/arch/s390/include/asm/gmap.h b/arch/s390/include/asm/gmap.h
> index 746e18bf8984..2a913014f63c 100644
> --- a/arch/s390/include/asm/gmap.h
> +++ b/arch/s390/include/asm/gmap.h
> @@ -147,7 +147,41 @@ 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_remove_old_asce(struct gmap *gmap);
> int s390_replace_asce(struct gmap *gmap);
> +void s390_uv_destroy_pfns(unsigned long count, unsigned long *pfns);
> +int __s390_uv_destroy_range(struct mm_struct *mm, unsigned long start,
> + unsigned long end, int interruptible);

s/int/bool/ ?

> +
> +/**
> + * s390_uv_destroy_range - Destroy a range of pages in the given mm.
> + * @mm the mm on which to operate on
> + * @start the start of the range
> + * @end the end of the range
> + *
> + * This call will call cond_sched, so it should not generate stalls, but it

This function will call ?

> + * will otherwise only return when it completed.
> + */
> +static inline void s390_uv_destroy_range(struct mm_struct *mm, unsigned long start,
> + unsigned long end)
> +{
> + (void)__s390_uv_destroy_range(mm, start, end, 0);
> +}
> +
> +/**
> + * s390_uv_destroy_range_interruptibe - Destroy a range of pages in the

interruptible

> + * given mm, but stop when a fatal signal is received.
> + * @mm the mm on which to operate on
> + * @start the start of the range
> + * @end the end of the range
> + *
> + * This call will call cond_sched, so it should not generate stalls. It
> + * will return -EINTR if a fatal signal is received, or 0 if the whole range
> + * has been destroyed.
> + */
> +static inline int s390_uv_destroy_range_interruptible(struct mm_struct *mm, unsigned long start,
> + unsigned long end)
> +{
> + return __s390_uv_destroy_range(mm, start, end, 1);
> +}
> #endif /* _ASM_S390_GMAP_H */
> diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
> index 3c59ef763dde..2ab22500e092 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)
> @@ -157,8 +159,14 @@ 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, 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 ce6cac4463f2..6eb5acb4be3d 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -2676,44 +2676,81 @@ 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];
> +};
> +
> +static int s390_gather_pages(pte_t *ptep, unsigned long addr,
> + unsigned long next, struct mm_walk *walk)
> {
> + 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);
> +
> +/**
> + * __s390_uv_destroy_range - Walk the given range of the given address
> + * space, and call the destroy secure page UVC on each page.
> + * Optionally exit early if a fatal signal is pending.
> + * @mm the mm to operate on
> + * @start the start of the range
> + * @end the end of the range
> + * @interruptible if not 0, stop when a fatal signal is received
> + * Return: 0 on success, -EINTR if the function stopped before completing
> + */
> +int __s390_uv_destroy_range(struct mm_struct *mm, unsigned long start,
> + unsigned long end, int interruptible)
> +{
> + struct reset_walk_state state = { .next = start };
> + int r = 1;
> +
> + while (r > 0) {
> + 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);
> + if (interruptible && fatal_signal_pending(current))
> + return -EINTR;
> + }
> + return 0;
> }
> -EXPORT_SYMBOL_GPL(s390_reset_acc);
> +EXPORT_SYMBOL_GPL(__s390_uv_destroy_range);
>
> /**
> * s390_remove_old_asce - Remove the topmost level of page tables from the
>