linux-kernel - Re: [PATCH v9 04/18] KVM: s390: pv: refactor s390_reset

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Date:   Fri, 1 Apr 2022 16:25:12 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Janosch Frank <frankja@...ux.ibm.com>
Cc:     kvm@...r.kernel.org, borntraeger@...ibm.com, thuth@...hat.com,
        pasic@...ux.ibm.com, david@...hat.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, scgl@...ux.ibm.com,
        mimu@...ux.ibm.com, nrb@...ux.ibm.com
Subject: Re: [PATCH v9 04/18] KVM: s390: pv: refactor s390_reset_acc

On Thu, 31 Mar 2022 15:25:31 +0200
Janosch Frank <frankja@...ux.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@...ux.ibm.com>
> > (dropping Janosch's Ack because of major changes to the patch)  
> 
> Reviewed-by: Janosch Frank <frankja@...ux.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  
>