linux-kernel - Re: [PATCH 6/9] mm/rmap: use rmap_walk() in try_to

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Date:	Mon, 2 Dec 2013 15:01:07 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Hillf Danton <dhillf@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Joonsoo Kim <js1304@...il.com>
Subject: Re: [PATCH 6/9] mm/rmap: use rmap_walk() in try_to_unmap()

On Thu, 28 Nov 2013 16:48:43 +0900 Joonsoo Kim <iamjoonsoo.kim@....com> wrote:

> Now, we have an infrastructure in rmap_walk() to handle difference
> from variants of rmap traversing functions.
> 
> So, just use it in try_to_unmap().
> 
> In this patch, I change following things.
> 
> 1. enable rmap_walk() if !CONFIG_MIGRATION.
> 2. mechanical change to use rmap_walk() in try_to_unmap().
> 
> ...
>
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -190,7 +190,7 @@ int page_referenced_one(struct page *, struct vm_area_struct *,
>  
>  int try_to_unmap(struct page *, enum ttu_flags flags);
>  int try_to_unmap_one(struct page *, struct vm_area_struct *,
> -			unsigned long address, enum ttu_flags flags);
> +			unsigned long address, void *arg);

This change is ugly and unchangelogged.

Also, "enum ttu_flags flags" was nice and meaningful, but "void *arg"
conveys far less information.  A suitable way to address this
shortcoming is to document `arg' at the try_to_unmap_one() definition
site.  try_to_unmap_one() doesn't actually have any documentation at
this stage - let's please fix that?

>
> ...
>
> @@ -1509,6 +1510,11 @@ bool is_vma_temporary_stack(struct vm_area_struct *vma)
>  	return false;
>  }
>  
> +static int skip_vma_temporary_stack(struct vm_area_struct *vma, void *arg)
> +{
> +	return (int)is_vma_temporary_stack(vma);
> +}

The (int) cast is unneeded - the compiler will turn a bool into an int.

Should this function (and rmap_walk_control.skip()) really be returning
a bool?

The name of this function is poor: "skip_foo" implies that the function
will skip over a foo.  But that isn't what this function does.  Please
choose something which accurately reflects the function's behavior.

>  /**
>   * try_to_unmap_anon - unmap or unlock anonymous page using the object-based
>   * rmap method
> @@ -1554,7 +1560,7 @@ static int try_to_unmap_anon(struct page *page, enum ttu_flags flags)
>  			continue;
>  
>  		address = vma_address(page, vma);
> -		ret = try_to_unmap_one(page, vma, address, flags);
> +		ret = try_to_unmap_one(page, vma, address, (void *)flags);
>  		if (ret != SWAP_AGAIN || !page_mapped(page))
>  			break;
>  	}
>
> ...
>
>  /**
>   * try_to_unmap - try to remove all page table mappings to a page
>   * @page: the page to get unmapped
> @@ -1630,16 +1641,30 @@ out:
>  int try_to_unmap(struct page *page, enum ttu_flags flags)
>  {
>  	int ret;
> +	struct rmap_walk_control rwc;
>  
> -	BUG_ON(!PageLocked(page));
>  	VM_BUG_ON(!PageHuge(page) && PageTransHuge(page));
>  
> -	if (unlikely(PageKsm(page)))
> -		ret = try_to_unmap_ksm(page, flags);
> -	else if (PageAnon(page))
> -		ret = try_to_unmap_anon(page, flags);
> -	else
> -		ret = try_to_unmap_file(page, flags);
> +	memset(&rwc, 0, sizeof(rwc));
> +	rwc.main = try_to_unmap_one;
> +	rwc.arg = (void *)flags;
> +	rwc.main_done = page_not_mapped;
> +	rwc.file_nonlinear = try_to_unmap_nonlinear;
> +	rwc.anon_lock = page_lock_anon_vma_read;

	struct rmap_walk_control rwc = {
		...
	};

> +	/*
> +	 * During exec, a temporary VMA is setup and later moved.
> +	 * The VMA is moved under the anon_vma lock but not the
> +	 * page tables leading to a race where migration cannot
> +	 * find the migration ptes. Rather than increasing the
> +	 * locking requirements of exec(), migration skips
> +	 * temporary VMAs until after exec() completes.
> +	 */
> +	if (flags & TTU_MIGRATION && !PageKsm(page) && PageAnon(page))
> +		rwc.vma_skip = skip_vma_temporary_stack;
> +
> +	ret = rmap_walk(page, &rwc);
> +
>  	if (ret != SWAP_MLOCK && !page_mapped(page))
>  		ret = SWAP_SUCCESS;
>  	return ret;
>
> ...
>

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