linux-kernel - Re: [PATCH 2/8] mm: Use find_get_swap

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Date:   Wed, 26 Aug 2020 10:20:02 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        William Kucharski <william.kucharski@...cle.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Matthew Auld <matthew.auld@...el.com>,
        Huang Ying <ying.huang@...el.com>,
        intel-gfx@...ts.freedesktop.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/8] mm: Use find_get_swap_page in memcontrol

On Wed, Aug 19, 2020 at 07:48:44PM +0100, Matthew Wilcox (Oracle) wrote:
> The current code does not protect against swapoff of the underlying
> swap device, so this is a bug fix as well as a worthwhile reduction in
> code complexity.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  mm/memcontrol.c | 25 ++-----------------------
>  1 file changed, 2 insertions(+), 23 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b807952b4d43..4075f214a841 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5539,35 +5539,14 @@ static struct page *mc_handle_swap_pte(struct vm_area_struct *vma,
>  static struct page *mc_handle_file_pte(struct vm_area_struct *vma,
>  			unsigned long addr, pte_t ptent, swp_entry_t *entry)
>  {
> -	struct page *page = NULL;
> -	struct address_space *mapping;
> -	pgoff_t pgoff;
> -
>  	if (!vma->vm_file) /* anonymous vma */
>  		return NULL;
>  	if (!(mc.flags & MOVE_FILE))
>  		return NULL;
>  
> -	mapping = vma->vm_file->f_mapping;
> -	pgoff = linear_page_index(vma, addr);
> -
>  	/* page is moved even if it's not RSS of this task(page-faulted). */
> -#ifdef CONFIG_SWAP
> -	/* shmem/tmpfs may report page out on swap: account for that too. */
> -	if (shmem_mapping(mapping)) {
> -		page = find_get_entry(mapping, pgoff);
> -		if (xa_is_value(page)) {
> -			swp_entry_t swp = radix_to_swp_entry(page);
> -			*entry = swp;
> -			page = find_get_page(swap_address_space(swp),
> -					     swp_offset(swp));
> -		}
> -	} else
> -		page = find_get_page(mapping, pgoff);
> -#else
> -	page = find_get_page(mapping, pgoff);
> -#endif
> -	return page;
> +	return find_get_swap_page(vma->vm_file->f_mapping,
> +			linear_page_index(vma, addr));

The refactor makes sense to me, but the name is confusing. We're not
looking for a swap page, we're primarily looking for a file page in
the page cache mapping that's handed in. Only in the special case
where it's a shmem mapping and there is a swap entry do we consult the
auxiliary swap cache.

How about find_get_page_or_swapcache()? find_get_page_shmemswap()?
Maybe you have a better idea. It's a fairly specialized operation that
isn't widely used, so a longer name isn't a bad thing IMO.