linux-kernel - Re: [RFC 08/11] mm: make ttu's return boolean

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Date:   Tue, 7 Mar 2017 23:13:26 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     <kernel-team@....com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [RFC 08/11] mm: make ttu's return boolean

On 03/01/2017 10:39 PM, Minchan Kim wrote:
> try_to_unmap returns SWAP_SUCCESS or SWAP_FAIL so it's suitable for
> boolean return. This patch changes it.

Hi Minchan,

So, up until this patch, I definitely like the cleanup, because as you observed, the 
return values didn't need so many different values. However, at this point, I think 
you should stop, and keep the SWAP_SUCCESS and SWAP_FAIL (or maybe even rename them 
to UNMAP_* or TTU_RESULT_*, to match their functions' names better), because 
removing them makes the code considerably less readable.

And since this is billed as a cleanup, we care here, even though this is a minor 
point. :)

Bool return values are sometimes perfect, such as when asking a question:

    bool mode_changed = needs_modeset(crtc_state);

The above is very nice. However, for returning success or failure, bools are not as 
nice, because *usually* success == true, except when you use the errno-based system, 
in which success == 0 (which would translate to false, if you mistakenly treated it 
as a bool). That leads to the reader having to remember which system is in use, 
usually with no visual cues to help.

>
[...]
>  	if (PageSwapCache(p)) {
> @@ -971,7 +971,7 @@ static int hwpoison_user_mappings(struct page *p, unsigned long pfn,
>  		collect_procs(hpage, &tokill, flags & MF_ACTION_REQUIRED);
>
>  	ret = try_to_unmap(hpage, ttu);
> -	if (ret != SWAP_SUCCESS)
> +	if (!ret)
>  		pr_err("Memory failure: %#lx: failed to unmap page (mapcount=%d)\n",
>  		       pfn, page_mapcount(hpage));
>
> @@ -986,8 +986,7 @@ static int hwpoison_user_mappings(struct page *p, unsigned long pfn,
>  	 * any accesses to the poisoned memory.
>  	 */
>  	forcekill = PageDirty(hpage) || (flags & MF_MUST_KILL);
> -	kill_procs(&tokill, forcekill, trapno,
> -		      ret != SWAP_SUCCESS, p, pfn, flags);
> +	kill_procs(&tokill, forcekill, trapno, !ret , p, pfn, flags);

The kill_procs() invocation was a little more readable before.

>
[...]
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 170c61f..e4b74f1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -966,7 +966,6 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		int may_enter_fs;
>  		enum page_references references = PAGEREF_RECLAIM_CLEAN;
>  		bool dirty, writeback;
> -		int ret = SWAP_SUCCESS;
>
>  		cond_resched();
>
> @@ -1139,13 +1138,9 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		 * processes. Try to unmap it here.
>  		 */
>  		if (page_mapped(page)) {
> -			switch (ret = try_to_unmap(page,
> -				ttu_flags | TTU_BATCH_FLUSH)) {
> -			case SWAP_FAIL:

Again: the SWAP_FAIL makes it crystal clear which case we're in.

I also wonder if UNMAP_FAIL or TTU_RESULT_FAIL is a better name?

thanks,
John Hubbard
NVIDIA