linux-kernel - Re: [PATCH v3 2/2] s390/mm: re-enable the shared zeropage for !PV and !skeys KVM guests

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Date: Mon, 15 Apr 2024 20:24:21 +0200
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>, Sven Schnelle <svens@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH v3 2/2] s390/mm: re-enable the shared zeropage for !PV
 and !skeys KVM guests

On Thu, Apr 11, 2024 at 06:14:41PM +0200, David Hildenbrand wrote:

David, could you please clarify the below questions?

> +static int __s390_unshare_zeropages(struct mm_struct *mm)
> +{
> +	struct vm_area_struct *vma;
> +	VMA_ITERATOR(vmi, mm, 0);
> +	unsigned long addr;
> +	int rc;
> +
> +	for_each_vma(vmi, vma) {
> +		/*
> +		 * We could only look at COW mappings, but it's more future
> +		 * proof to catch unexpected zeropages in other mappings and
> +		 * fail.
> +		 */
> +		if ((vma->vm_flags & VM_PFNMAP) || is_vm_hugetlb_page(vma))
> +			continue;
> +		addr = vma->vm_start;
> +
> +retry:
> +		rc = walk_page_range_vma(vma, addr, vma->vm_end,
> +					 &find_zeropage_ops, &addr);
> +		if (rc <= 0)
> +			continue;

So in case an error is returned for the last vma, __s390_unshare_zeropage()
finishes with that error. By contrast, the error for a non-last vma would
be ignored?

> +
> +		/* addr was updated by find_zeropage_pte_entry() */
> +		rc = handle_mm_fault(vma, addr,
> +				     FAULT_FLAG_UNSHARE | FAULT_FLAG_REMOTE,
> +				     NULL);
> +		if (rc & VM_FAULT_OOM)
> +			return -ENOMEM;

Heiko pointed out that rc type is inconsistent vs vm_fault_t returned by
handle_mm_fault(). While fixing it up, I've got concerned whether is it
fine to continue in case any other error is met (including possible future
VM_FAULT_xxxx)?

> +		/*
> +		 * See break_ksm(): even after handle_mm_fault() returned 0, we
> +		 * must start the lookup from the current address, because
> +		 * handle_mm_fault() may back out if there's any difficulty.
> +		 *
> +		 * VM_FAULT_SIGBUS and VM_FAULT_SIGSEGV are unexpected but
> +		 * maybe they could trigger in the future on concurrent
> +		 * truncation. In that case, the shared zeropage would be gone
> +		 * and we can simply retry and make progress.
> +		 */
> +		cond_resched();
> +		goto retry;
> +	}
> +
> +	return rc;
> +}

Thanks!