linux-kernel - Re: [PATCH v7 1/8] mm: Remove special swap entry functions

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Message-ID: <20210330183833.GY2356281@nvidia.com>
Date:   Tue, 30 Mar 2021 15:38:33 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, nouveau@...ts.freedesktop.org,
        bskeggs@...hat.com, akpm@...ux-foundation.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm-ppc@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        jhubbard@...dia.com, rcampbell@...dia.com, jglisse@...hat.com,
        hch@...radead.org, daniel@...ll.ch, willy@...radead.org,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v7 1/8] mm: Remove special swap entry functions

On Fri, Mar 26, 2021 at 11:07:58AM +1100, Alistair Popple wrote:
> Remove multiple similar inline functions for dealing with different
> types of special swap entries.
> 
> Both migration and device private swap entries use the swap offset to
> store a pfn. Instead of multiple inline functions to obtain a struct
> page for each swap entry type use a common function
> pfn_swap_entry_to_page(). Also open-code the various entry_to_pfn()
> functions as this results is shorter code that is easier to understand.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> Reviewed-by: Ralph Campbell <rcampbell@...dia.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> 
> ---
> 
> v7:
> * Reworded commit message to include pfn_swap_entry_to_page()
> * Added Christoph's Reviewed-by
> 
> v6:
> * Removed redundant compound_page() call from inside PageLocked()
> * Fixed a minor build issue for s390 reported by kernel test bot
> 
> v4:
> * Added pfn_swap_entry_to_page()
> * Reinstated check that migration entries point to locked pages
> * Removed #define swapcache_prepare which isn't needed for CONFIG_SWAP=0
>   builds
> ---
>  arch/s390/mm/pgtable.c  |  2 +-
>  fs/proc/task_mmu.c      | 23 +++++---------
>  include/linux/swap.h    |  4 +--
>  include/linux/swapops.h | 69 ++++++++++++++---------------------------
>  mm/hmm.c                |  5 ++-
>  mm/huge_memory.c        |  4 +--
>  mm/memcontrol.c         |  2 +-
>  mm/memory.c             | 10 +++---
>  mm/migrate.c            |  6 ++--
>  mm/page_vma_mapped.c    |  6 ++--
>  10 files changed, 50 insertions(+), 81 deletions(-)

Looks good

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

> diff --git a/mm/hmm.c b/mm/hmm.c
> index 943cb2ba4442..3b2dda71d0ed 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -214,7 +214,7 @@ static inline bool hmm_is_device_private_entry(struct hmm_range *range,
>  		swp_entry_t entry)
>  {
>  	return is_device_private_entry(entry) &&
> -		device_private_entry_to_page(entry)->pgmap->owner ==
> +		pfn_swap_entry_to_page(entry)->pgmap->owner ==
>  		range->dev_private_owner;
>  }
>  
> @@ -257,8 +257,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
>  			cpu_flags = HMM_PFN_VALID;
>  			if (is_write_device_private_entry(entry))
>  				cpu_flags |= HMM_PFN_WRITE;
> -			*hmm_pfn = device_private_entry_to_pfn(entry) |
> -					cpu_flags;
> +			*hmm_pfn = swp_offset(entry) | cpu_flags;

Though swp_offset() seems poor here

Something like this seems nicer, maybe as an additional patch in this
series?

diff --git a/mm/hmm.c b/mm/hmm.c
index 943cb2ba444232..c06cbc4e3981b7 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -210,14 +210,6 @@ int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
 		unsigned long end, unsigned long hmm_pfns[], pmd_t pmd);
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
-static inline bool hmm_is_device_private_entry(struct hmm_range *range,
-		swp_entry_t entry)
-{
-	return is_device_private_entry(entry) &&
-		device_private_entry_to_page(entry)->pgmap->owner ==
-		range->dev_private_owner;
-}
-
 static inline unsigned long pte_to_hmm_pfn_flags(struct hmm_range *range,
 						 pte_t pte)
 {
@@ -226,6 +218,32 @@ static inline unsigned long pte_to_hmm_pfn_flags(struct hmm_range *range,
 	return pte_write(pte) ? (HMM_PFN_VALID | HMM_PFN_WRITE) : HMM_PFN_VALID;
 }
 
+static bool hmm_pte_handle_device_private(struct hmm_range *range, pte_t pte,
+					  unsigned long *hmm_pfn)
+{
+	swp_entry_t entry = pte_to_swp_entry(pte);
+	struct page *device_page;
+	unsigned long cpu_flags;
+
+	if (is_device_private_entry(entry))
+		return false;
+
+	/*
+	 * If the device private page matches the device the caller understands
+	 * then return the private pfn directly. The caller must know what to do
+	 * with it.
+	 */
+	device_page = pfn_swap_entry_to_page(entry);
+	if (device_page->pgmap->owner != range->dev_private_owner)
+		return false;
+
+	cpu_flags = HMM_PFN_VALID;
+	if (is_write_device_private_entry(entry))
+		cpu_flags |= HMM_PFN_WRITE;
+	*hmm_pfn = page_to_pfn(device_page) | cpu_flags;
+	return true;
+}
+
 static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 			      unsigned long end, pmd_t *pmdp, pte_t *ptep,
 			      unsigned long *hmm_pfn)
@@ -247,20 +265,8 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 	}
 
 	if (!pte_present(pte)) {
-		swp_entry_t entry = pte_to_swp_entry(pte);
-
-		/*
-		 * Never fault in device private pages, but just report
-		 * the PFN even if not present.
-		 */
-		if (hmm_is_device_private_entry(range, entry)) {
-			cpu_flags = HMM_PFN_VALID;
-			if (is_write_device_private_entry(entry))
-				cpu_flags |= HMM_PFN_WRITE;
-			*hmm_pfn = device_private_entry_to_pfn(entry) |
-					cpu_flags;
+		if (hmm_pte_handle_device_private(range, pte, hmm_pfn))
 			return 0;
-		}
 
 		required_fault =
 			hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0);

Jason