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Date:   Thu, 28 Mar 2019 06:30:12 -0700
From:   Ira Weiny <ira.weiny@intel.com>
To:     jglisse@redhat.com
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH v2 04/11] mm/hmm: improve and rename hmm_vma_get_pfns()
 to hmm_range_snapshot() v2
Message-ID: <20190328133012.GC31324@iweiny-DESK2.sc.intel.com>
References: <20190325144011.10560-1-jglisse@redhat.com>
 <20190325144011.10560-5-jglisse@redhat.com>
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On Mon, Mar 25, 2019 at 10:40:04AM -0400, Jerome Glisse wrote:
> From: J?r?me Glisse <jglisse@redhat.com>
> 
> Rename for consistency between code, comments and documentation. Also
> improves the comments on all the possible returns values. Improve the
> function by returning the number of populated entries in pfns array.
> 
> Changes since v1:
>     - updated documentation
>     - reformated some comments
> 
> Signed-off-by: J?r?me Glisse <jglisse@redhat.com>
> Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
> Reviewed-by: John Hubbard <jhubbard@nvidia.com>

Reviewed-by: Ira Weiny <ira.weiny@intel.com>

> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Dan Williams <dan.j.williams@intel.com>
> ---
>  Documentation/vm/hmm.rst | 26 ++++++++++++++++++--------
>  include/linux/hmm.h      |  4 ++--
>  mm/hmm.c                 | 31 +++++++++++++++++--------------
>  3 files changed, 37 insertions(+), 24 deletions(-)
> 
> diff --git a/Documentation/vm/hmm.rst b/Documentation/vm/hmm.rst
> index 44205f0b671f..d9b27bdadd1b 100644
> --- a/Documentation/vm/hmm.rst
> +++ b/Documentation/vm/hmm.rst
> @@ -189,11 +189,7 @@ the driver callback returns.
>  When the device driver wants to populate a range of virtual addresses, it can
>  use either::
>  
> -  int hmm_vma_get_pfns(struct vm_area_struct *vma,
> -                      struct hmm_range *range,
> -                      unsigned long start,
> -                      unsigned long end,
> -                      hmm_pfn_t *pfns);
> +  long hmm_range_snapshot(struct hmm_range *range);
>    int hmm_vma_fault(struct vm_area_struct *vma,
>                      struct hmm_range *range,
>                      unsigned long start,
> @@ -202,7 +198,7 @@ When the device driver wants to populate a range of virtual addresses, it can
>                      bool write,
>                      bool block);
>  
> -The first one (hmm_vma_get_pfns()) will only fetch present CPU page table
> +The first one (hmm_range_snapshot()) will only fetch present CPU page table
>  entries and will not trigger a page fault on missing or non-present entries.
>  The second one does trigger a page fault on missing or read-only entry if the
>  write parameter is true. Page faults use the generic mm page fault code path
> @@ -220,19 +216,33 @@ Locking with the update() callback is the most important aspect the driver must
>   {
>        struct hmm_range range;
>        ...
> +
> +      range.start = ...;
> +      range.end = ...;
> +      range.pfns = ...;
> +      range.flags = ...;
> +      range.values = ...;
> +      range.pfn_shift = ...;
> +
>   again:
> -      ret = hmm_vma_get_pfns(vma, &range, start, end, pfns);
> -      if (ret)
> +      down_read(&mm->mmap_sem);
> +      range.vma = ...;
> +      ret = hmm_range_snapshot(&range);
> +      if (ret) {
> +          up_read(&mm->mmap_sem);
>            return ret;
> +      }
>        take_lock(driver->update);
>        if (!hmm_vma_range_done(vma, &range)) {
>            release_lock(driver->update);
> +          up_read(&mm->mmap_sem);
>            goto again;
>        }
>  
>        // Use pfns array content to update device page table
>  
>        release_lock(driver->update);
> +      up_read(&mm->mmap_sem);
>        return 0;
>   }
>  
> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> index 716fc61fa6d4..32206b0b1bfd 100644
> --- a/include/linux/hmm.h
> +++ b/include/linux/hmm.h
> @@ -365,11 +365,11 @@ void hmm_mirror_unregister(struct hmm_mirror *mirror);
>   * table invalidation serializes on it.
>   *
>   * YOU MUST CALL hmm_vma_range_done() ONCE AND ONLY ONCE EACH TIME YOU CALL
> - * hmm_vma_get_pfns() WITHOUT ERROR !
> + * hmm_range_snapshot() WITHOUT ERROR !
>   *
>   * IF YOU DO NOT FOLLOW THE ABOVE RULE THE SNAPSHOT CONTENT MIGHT BE INVALID !
>   */
> -int hmm_vma_get_pfns(struct hmm_range *range);
> +long hmm_range_snapshot(struct hmm_range *range);
>  bool hmm_vma_range_done(struct hmm_range *range);
>  
>  
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 213b0beee8d3..91361aa74b8b 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -698,23 +698,25 @@ static void hmm_pfns_special(struct hmm_range *range)
>  }
>  
>  /*
> - * hmm_vma_get_pfns() - snapshot CPU page table for a range of virtual addresses
> - * @range: range being snapshotted
> - * Returns: -EINVAL if invalid argument, -ENOMEM out of memory, -EPERM invalid
> - *          vma permission, 0 success
> + * hmm_range_snapshot() - snapshot CPU page table for a range
> + * @range: range
> + * Returns: number of valid pages in range->pfns[] (from range start
> + *          address). This may be zero. If the return value is negative,
> + *          then one of the following values may be returned:
> + *
> + *           -EINVAL  invalid arguments or mm or virtual address are in an
> + *                    invalid vma (ie either hugetlbfs or device file vma).
> + *           -EPERM   For example, asking for write, when the range is
> + *                    read-only
> + *           -EAGAIN  Caller needs to retry
> + *           -EFAULT  Either no valid vma exists for this range, or it is
> + *                    illegal to access the range
>   *
>   * This snapshots the CPU page table for a range of virtual addresses. Snapshot
>   * validity is tracked by range struct. See hmm_vma_range_done() for further
>   * information.
> - *
> - * The range struct is initialized here. It tracks the CPU page table, but only
> - * if the function returns success (0), in which case the caller must then call
> - * hmm_vma_range_done() to stop CPU page table update tracking on this range.
> - *
> - * NOT CALLING hmm_vma_range_done() IF FUNCTION RETURNS 0 WILL LEAD TO SERIOUS
> - * MEMORY CORRUPTION ! YOU HAVE BEEN WARNED !
>   */
> -int hmm_vma_get_pfns(struct hmm_range *range)
> +long hmm_range_snapshot(struct hmm_range *range)
>  {
>  	struct vm_area_struct *vma = range->vma;
>  	struct hmm_vma_walk hmm_vma_walk;
> @@ -768,6 +770,7 @@ int hmm_vma_get_pfns(struct hmm_range *range)
>  	hmm_vma_walk.fault = false;
>  	hmm_vma_walk.range = range;
>  	mm_walk.private = &hmm_vma_walk;
> +	hmm_vma_walk.last = range->start;
>  
>  	mm_walk.vma = vma;
>  	mm_walk.mm = vma->vm_mm;
> @@ -784,9 +787,9 @@ int hmm_vma_get_pfns(struct hmm_range *range)
>  	 * function return 0).
>  	 */
>  	range->hmm = hmm;
> -	return 0;
> +	return (hmm_vma_walk.last - range->start) >> PAGE_SHIFT;
>  }
> -EXPORT_SYMBOL(hmm_vma_get_pfns);
> +EXPORT_SYMBOL(hmm_range_snapshot);
>  
>  /*
>   * hmm_vma_range_done() - stop tracking change to CPU page table over a range
> -- 
> 2.17.2
>