linux-kernel - Re: [PATCH v2 RESEND 2/4] Drivers: hv: balloon: account for gaps in hot add regions

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Date:	Tue, 16 Aug 2016 10:40:08 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	"Alex Ng \(LIS\)" <alexng@...rosoft.com>
Cc:	"devel\@linuxdriverproject.org" <devel@...uxdriverproject.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"KY Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>
Subject: Re: [PATCH v2 RESEND 2/4] Drivers: hv: balloon: account for gaps in hot add regions

"Alex Ng (LIS)" <alexng@...rosoft.com> writes:

>> @@ -676,35 +686,63 @@ static void hv_mem_hot_add(unsigned long start,
>> unsigned long size,
>> 
>>  static void hv_online_page(struct page *pg)  {
>> -	struct list_head *cur;
>>  	struct hv_hotadd_state *has;
>> +	struct hv_hotadd_gap *gap;
>>  	unsigned long cur_start_pgp;
>>  	unsigned long cur_end_pgp;
>> +	bool is_gap = false;
>> 
>>  	list_for_each(cur, &dm_device.ha_region_list) {
>>  		has = list_entry(cur, struct hv_hotadd_state, list);
>>  		cur_start_pgp = (unsigned long)
>> +			pfn_to_page(has->start_pfn);
>> +		cur_end_pgp = (unsigned long)pfn_to_page(has->end_pfn);
>> +
>> +		/* The page belongs to a different HAS. */
>> +		if (((unsigned long)pg < cur_start_pgp) ||
>> +		    ((unsigned long)pg >= cur_end_pgp))
>> +			continue;
>> +
>> +		cur_start_pgp = (unsigned long)
>>  			pfn_to_page(has->covered_start_pfn);
>>  		cur_end_pgp = (unsigned long)pfn_to_page(has-
>> >covered_end_pfn);
>> 
>> -		if (((unsigned long)pg >= cur_start_pgp) &&
>> -			((unsigned long)pg < cur_end_pgp)) {
>> -			/*
>> -			 * This frame is currently backed; online the
>> -			 * page.
>> -			 */
>> -			__online_page_set_limits(pg);
>> -			__online_page_increment_counters(pg);
>> -			__online_page_free(pg);
>> +		/* The page is not backed. */
>> +		if (((unsigned long)pg < cur_start_pgp) ||
>> +		    ((unsigned long)pg >= cur_end_pgp))
>> +			continue;
>> +
>> +		/* Check for gaps. */
>> +		list_for_each_entry(gap, &has->gap_list, list) {
>> +			cur_start_pgp = (unsigned long)
>> +				pfn_to_page(gap->start_pfn);
>> +			cur_end_pgp = (unsigned long)
>> +				pfn_to_page(gap->end_pfn);
>> +			if (((unsigned long)pg >= cur_start_pgp) &&
>> +			    ((unsigned long)pg < cur_end_pgp)) {
>> +				is_gap = true;
>> +				break;
>> +			}
>>  		}
>> +
>> +		if (is_gap)
>> +			break;
>> +
>> +		/* This frame is currently backed; online the page. */
>> +		__online_page_set_limits(pg);
>> +		__online_page_increment_counters(pg);
>> +		__online_page_free(pg);
>> +		break;
>>  	}
>>  }
>> 
>
> We may need to add similar logic to check for gaps in hv_bring_pgs_online().
>
> [...]

Yes, probably, I'll take a look and try to refactor the onlinig code in
a separate function to avoid duplication.

>>  static unsigned long handle_pg_range(unsigned long pg_start, @@ -834,13
>> +881,19 @@ static unsigned long process_hot_add(unsigned long pg_start,
>>  					unsigned long rg_size)
>>  {
>>  	struct hv_hotadd_state *ha_region = NULL;
>> +	int covered;
>> 
>>  	if (pfn_cnt == 0)
>>  		return 0;
>> 
>> -	if (!dm_device.host_specified_ha_region)
>> -		if (pfn_covered(pg_start, pfn_cnt))
>> +	if (!dm_device.host_specified_ha_region) {
>> +		covered = pfn_covered(pg_start, pfn_cnt);
>> +		if (covered < 0)
>> +			return 0;
>
> If the hot-add pages aren't covered by any region, then shouldn't it fall through instead of returning?
> That way the new ha_region can be added to the list and we hot-add the
> pages accordingly.

I was under an impression this is impossible:
hot_add_req()/process_hot_add() will create a new region in this
case. 'covered < 0' was added to handle one particular error: failure to
allocate memory to record gap (struct hv_hotadd_gap) and I don't have a
better idea how to handle this: if we can't remember the gap we'll crash
later on onlining...

-- 
  Vitaly