linux-kernel - Re: [PATCH 10/15] mm/hmm: do not differentiate between empty entry or missing directory v2

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Date:   Tue, 20 Mar 2018 22:24:34 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     <jglisse@...hat.com>, <linux-mm@...ck.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        <linux-kernel@...r.kernel.org>,
        Evgeny Baskakov <ebaskakov@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Mark Hairgrove <mhairgrove@...dia.com>
Subject: Re: [PATCH 10/15] mm/hmm: do not differentiate between empty entry or
 missing directory v2

On 03/19/2018 07:00 PM, jglisse@...hat.com wrote:
> From: Jérôme Glisse <jglisse@...hat.com>
> 
> There is no point in differentiating between a range for which there
> is not even a directory (and thus entries) and empty entry (pte_none()
> or pmd_none() returns true).
> 
> Simply drop the distinction ie remove HMM_PFN_EMPTY flag and merge now
> duplicate hmm_vma_walk_hole() and hmm_vma_walk_clear() functions.
> 
> Changed since v1:
>   - Improved comments
> 
> Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
> Cc: Evgeny Baskakov <ebaskakov@...dia.com>
> Cc: Ralph Campbell <rcampbell@...dia.com>
> Cc: Mark Hairgrove <mhairgrove@...dia.com>
> Cc: John Hubbard <jhubbard@...dia.com>
> ---
>  include/linux/hmm.h |  8 +++-----
>  mm/hmm.c            | 45 +++++++++++++++------------------------------
>  2 files changed, 18 insertions(+), 35 deletions(-)
> 
> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> index 54d684fe3b90..cf283db22106 100644
> --- a/include/linux/hmm.h
> +++ b/include/linux/hmm.h
> @@ -84,7 +84,6 @@ struct hmm;
>   * HMM_PFN_VALID: pfn is valid. It has, at least, read permission.
>   * 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()
>   * HMM_PFN_SPECIAL: corresponding CPU page table entry is special; i.e., the
>   *      result of vm_insert_pfn() or vm_insert_page(). Therefore, it should not
>   *      be mirrored by a device, because the entry will never have HMM_PFN_VALID
> @@ -94,10 +93,9 @@ struct hmm;
>  #define HMM_PFN_VALID (1 << 0)
>  #define HMM_PFN_WRITE (1 << 1)
>  #define HMM_PFN_ERROR (1 << 2)
> -#define HMM_PFN_EMPTY (1 << 3)

Hi Jerome,

Nearly done with this one...see below for a bit more detail, but I think if we did this:

    #define HMM_PFN_EMPTY (0)

...it would work out nicely.

> -#define HMM_PFN_SPECIAL (1 << 4)
> -#define HMM_PFN_DEVICE_UNADDRESSABLE (1 << 5)
> -#define HMM_PFN_SHIFT 6
> +#define HMM_PFN_SPECIAL (1 << 3)
> +#define HMM_PFN_DEVICE_UNADDRESSABLE (1 << 4)
> +#define HMM_PFN_SHIFT 5
>  

<snip>

> @@ -438,7 +423,7 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>  		pfns[i] = 0;
>  
>  		if (pte_none(pte)) {
> -			pfns[i] = HMM_PFN_EMPTY;
> +			pfns[i] = 0;

This works, but why not keep HMM_PFN_EMPTY, and just define it as zero?
Symbols are better than raw numbers here.


>  			if (hmm_vma_walk->fault)
>  				goto fault;
>  			continue;
> @@ -489,8 +474,8 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>  
>  fault:
>  		pte_unmap(ptep);
> -		/* Fault all pages in range */
> -		return hmm_vma_walk_clear(start, end, walk);
> +		/* Fault any virtual address we were ask to fault */

                                                     asked to fault

thanks,
-- 
John Hubbard
NVIDIA