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

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