Re: [PATCH 06/14] mm/hmm: remove HMM_PFN_READ flag and ignore peculiar architecture

[John Hubbard <jhubbard@nvidia.com>]

On 03/16/2018 12:14 PM, jglisse@redhat.com wrote:
> From: Jérôme Glisse <jglisse@redhat.com>
> 
> Only peculiar architecture allow write without read thus assume that
> any valid pfn do allow for read. Note we do not care for write only
> because it does make sense with thing like atomic compare and exchange
> or any other operations that allow you to get the memory value through
> them.
> 
> Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
> Cc: Evgeny Baskakov <ebaskakov@nvidia.com>
> Cc: Ralph Campbell <rcampbell@nvidia.com>
> Cc: Mark Hairgrove <mhairgrove@nvidia.com>
> Cc: John Hubbard <jhubbard@nvidia.com>
> ---
>  include/linux/hmm.h | 14 ++++++--------
>  mm/hmm.c            | 28 ++++++++++++++++++++++++----
>  2 files changed, 30 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> index b65e527dd120..4bdc58ffe9f3 100644
> --- a/include/linux/hmm.h
> +++ b/include/linux/hmm.h
> @@ -84,7 +84,6 @@ struct hmm;
>   *
>   * Flags:
>   * HMM_PFN_VALID: pfn is valid

Maybe write it like this:

* HMM_PFN_VALID: pfn is valid. This implies that it has, at least, read permission.

> - * HMM_PFN_READ:  CPU page table has read permission set
>   * HMM_PFN_WRITE: CPU page table has write permission set
>   * HMM_PFN_ERROR: corresponding CPU page table entry points to poisoned memory
>   * HMM_PFN_EMPTY: corresponding CPU page table entry is pte_none()
> @@ -97,13 +96,12 @@ struct hmm;
>  typedef unsigned long hmm_pfn_t;
>  
>  #define HMM_PFN_VALID (1 << 0)

<snip>

>  
> @@ -536,6 +534,17 @@ int hmm_vma_get_pfns(struct hmm_range *range)
>  	list_add_rcu(&range->list, &hmm->ranges);
>  	spin_unlock(&hmm->lock);
>  
> +	if (!(vma->vm_flags & VM_READ)) {
> +		/*
> +		 * If vma do not allow read assume it does not allow write as
> +		 * only peculiar architecture allow write without read and this
> +		 * is not a case we care about (some operation like atomic no
> +		 * longer make sense).
> +		 */
> +		hmm_pfns_clear(range->pfns, range->start, range->end);
> +		return 0;

1. Shouldn't we return an error here? All is not well. No one has any pfns, even
   though they tried to get some. :)

2. I think this check needs to be done much earlier, right after the "Sanity
   check, this should not happen" code in this routine.

> +	}
> +
>  	hmm_vma_walk.fault = false;
>  	hmm_vma_walk.range = range;
>  	mm_walk.private = &hmm_vma_walk;
> @@ -690,6 +699,17 @@ int hmm_vma_fault(struct hmm_range *range, bool write, bool block)
>  	list_add_rcu(&range->list, &hmm->ranges);
>  	spin_unlock(&hmm->lock);
>  
> +	if (!(vma->vm_flags & VM_READ)) {
> +		/*
> +		 * If vma do not allow read assume it does not allow write as
> +		 * only peculiar architecture allow write without read and this
> +		 * is not a case we care about (some operation like atomic no
> +		 * longer make sense).
> +		 */

For the comment wording (for this one, and the one above), how about:

/*
 * If the vma does not allow read access, then assume that 
 * it does not allow write access, either.
 */

...and then leave the more extensive explanation to the commit log. Or,
if we really want a longer explananation right here, then:

/*
 * If the vma does not allow read access, then assume that 
 * it does not allow write access, either. Architectures that
 * allow write without read access are not supported by HMM,
 * because operations such as atomic access would not work.
 */


> +		hmm_pfns_clear(range->pfns, range->start, range->end);
> +		return 0;
> +	}

Similar points as above: it seems like an error case, and the check should be right near 
the beginning of the function.

thanks,
-- 
John Hubbard
NVIDIA