linux-kernel - Re: [PATCH v3 4/8] mm/rmap: Split migration into its own function

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Message-ID: <E93F89E1-3CE2-4CA3-97D9-6BCED78E1001@nvidia.com>
Date:   Tue, 2 Mar 2021 17:08:15 -0500
From:   Zi Yan <ziy@...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>,
        <dri-devel@...ts.freedesktop.org>, <jhubbard@...dia.com>,
        <rcampbell@...dia.com>, <jglisse@...hat.com>, <jgg@...dia.com>,
        <hch@...radead.org>, <daniel@...ll.ch>
Subject: Re: [PATCH v3 4/8] mm/rmap: Split migration into its own function

On 26 Feb 2021, at 2:18, Alistair Popple wrote:

> Migration is currently implemented as a mode of operation for
> try_to_unmap_one() generally specified by passing the TTU_MIGRATION flag
> or in the case of splitting a huge anonymous page TTU_SPLIT_FREEZE.
>
> However it does not have much in common with the rest of the unmap
> functionality of try_to_unmap_one() and thus splitting it into a
> separate function reduces the complexity of try_to_unmap_one() making it
> more readable.
>
> Several simplifications can also be made in try_to_migrate_one() based
> on the following observations:
>
>  - All users of TTU_MIGRATION also set TTU_IGNORE_MLOCK.
>  - No users of TTU_MIGRATION ever set TTU_IGNORE_HWPOISON.
>  - No users of TTU_MIGRATION ever set TTU_BATCH_FLUSH.
>
> TTU_SPLIT_FREEZE is a special case of migration used when splitting an
> anonymous page. This is most easily dealt with by calling the correct
> function from unmap_page() in mm/huge_memory.c  - either
> try_to_migrate() for PageAnon or try_to_unmap().
>
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> ---
>  include/linux/rmap.h |   4 +-
>  mm/huge_memory.c     |  10 +-
>  mm/migrate.c         |   9 +-
>  mm/rmap.c            | 352 +++++++++++++++++++++++++++++++------------
>  4 files changed, 269 insertions(+), 106 deletions(-)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 7f1ee411bd7b..77fa17de51d7 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -86,8 +86,6 @@ struct anon_vma_chain {
>  };
>
>  enum ttu_flags {
> -	TTU_MIGRATION		= 0x1,	/* migration mode */
> -
>  	TTU_SPLIT_HUGE_PMD	= 0x4,	/* split huge PMD if any */

It implies freeze in try_to_migrate() and no freeze in try_to_unmap(). I think
we need some comments here, above try_to_migrate(), and above try_to_unmap()
to clarify the implication.

>  	TTU_IGNORE_MLOCK	= 0x8,	/* ignore mlock */
>  	TTU_IGNORE_HWPOISON	= 0x20,	/* corrupted page is recoverable */
> @@ -96,7 +94,6 @@ enum ttu_flags {
>  					 * do a final flush if necessary */
>  	TTU_RMAP_LOCKED		= 0x80,	/* do not grab rmap lock:
>  					 * caller holds it */
> -	TTU_SPLIT_FREEZE	= 0x100,		/* freeze pte under splitting thp */
>  };
>
>  #ifdef CONFIG_MMU
> @@ -193,6 +190,7 @@ static inline void page_dup_rmap(struct page *page, bool compound)
>  int page_referenced(struct page *, int is_locked,
>  			struct mem_cgroup *memcg, unsigned long *vm_flags);
>
> +bool try_to_migrate(struct page *page, enum ttu_flags flags);
>  bool try_to_unmap(struct page *, enum ttu_flags flags);
>
>  /* Avoid racy checks */
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d00b93dc2d9e..357052a4567b 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2351,16 +2351,16 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
>
>  static void unmap_page(struct page *page)
>  {
> -	enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK |
> -		TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
> +	enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
>  	bool unmap_success;
>
>  	VM_BUG_ON_PAGE(!PageHead(page), page);
>
>  	if (PageAnon(page))
> -		ttu_flags |= TTU_SPLIT_FREEZE;
> -
> -	unmap_success = try_to_unmap(page, ttu_flags);
> +		unmap_success = try_to_migrate(page, ttu_flags);
> +	else
> +		unmap_success = try_to_unmap(page, ttu_flags |
> +						TTU_IGNORE_MLOCK);

I think we need a comment here about why anonymous pages need try_to_migrate()
and others need try_to_unmap().

Thanks.

—
Best Regards,
Yan Zi

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