Re: [PATCH v9 04/18] KVM: s390: pv: refactor s390_reset_acc

Claudio Imbrenda
2022-04-01T14:25:27Z

On Thu, 31 Mar 2022 15:25:31 +0200
Janosch Frank <frankja@linux.ibm.com> wrote:

> On 3/30/22 14:25, 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)
>
> Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
>
> [...]
> > +#define DESTROY_LOOP_THRESHOLD 32
>
> A question out of curiosity:
> Is there any particular reason for the number?
> Have you tested other numbers and experienced a speedup/slowdown?

to be honest no, it just seemed a good tradeoff between size and
callback overhead

>
> > +
> > +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, bool 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
>