Re: [PATCH] kvm: Faults which trigger IO release the mmap_sem

[Paolo Bonzini]

Il 15/09/2014 22:11, Andres Lagar-Cavilla ha scritto:
> +	if (!locked) {
> +		BUG_ON(npages != -EBUSY);

VM_BUG_ON perhaps?

> @@ -1177,9 +1210,15 @@ static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
>  		npages = get_user_page_nowait(current, current->mm,
>  					      addr, write_fault, page);
>  		up_read(&current->mm->mmap_sem);
> -	} else
> -		npages = get_user_pages_fast(addr, 1, write_fault,
> -					     page);
> +	} else {
> +		/*
> +		 * By now we have tried gup_fast, and possible async_pf, and we
> +		 * are certainly not atomic. Time to retry the gup, allowing
> +		 * mmap semaphore to be relinquished in the case of IO.
> +		 */
> +		npages = kvm_get_user_page_retry(current, current->mm, addr,
> +						 write_fault, page);

This is a separate logical change.  Was this:

	down_read(&mm->mmap_sem);
	npages = get_user_pages(NULL, mm, addr, 1, 1, 0, NULL, NULL);
	up_read(&mm->mmap_sem);

the intention rather than get_user_pages_fast?

I think a first patch should introduce kvm_get_user_page_retry ("Retry a
fault after a gup with FOLL_NOWAIT.") and the second would add
FOLL_TRIED ("This properly relinquishes mmap semaphore if the
filemap/swap has to wait on page lock (and retries the gup to completion
after that").

Apart from this, the patch looks good.  The mm/ parts are minimal, so I
think it's best to merge it through the KVM tree with someone's Acked-by.

Paolo
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