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Subject: Re: [PATCH -mm -v4 04/21] mm, THP, swap: Support PMD swap mapping in
 swapcache_free_cluster()
To:     "Huang, Ying" <ying.huang@intel.com>
References: <20180622035151.6676-1-ying.huang@intel.com>
 <20180622035151.6676-5-ying.huang@intel.com>
 <dd7b3dd7-9e10-4b9f-b931-915298bfd627@linux.intel.com>
 <874lh7intc.fsf@yhuang-dev.intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Michal Hocko <mhocko@suse.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shaohua Li <shli@kernel.org>, Hugh Dickins <hughd@google.com>,
        Minchan Kim <minchan@kernel.org>,
        Rik van Riel <riel@redhat.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Zi Yan <zi.yan@cs.rutgers.edu>,
        Daniel Jordan <daniel.m.jordan@oracle.com>
From:   Dave Hansen <dave.hansen@linux.intel.com>
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Message-ID: <444d0718-8b89-5ef1-15c8-1bbbc6cb1bf3@linux.intel.com>
Date:   Tue, 10 Jul 2018 06:54:21 -0700
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On 07/09/2018 11:53 PM, Huang, Ying wrote:
> Dave Hansen <dave.hansen@linux.intel.com> writes:
>>> +#ifdef CONFIG_THP_SWAP
>>> +static inline int cluster_swapcount(struct swap_cluster_info *ci)
>>> +{
>>> +	if (!ci || !cluster_is_huge(ci))
>>> +		return 0;
>>> +
>>> +	return cluster_count(ci) - SWAPFILE_CLUSTER;
>>> +}
>>> +#else
>>> +#define cluster_swapcount(ci)			0
>>> +#endif
>>
>> Dumb questions, round 2:  On a CONFIG_THP_SWAP=n build, presumably,
>> cluster_is_huge()=0 always, so cluster_swapout() always returns 0.  Right?
>>
>> So, why the #ifdef?
> 
> #ifdef here is to reduce the code size for !CONFIG_THP_SWAP.

I'd just remove the !CONFIG_THP_SWAP version entirely.

>>> @@ -1288,24 +1301,30 @@ static void swapcache_free_cluster(swp_entry_t entry)
>>>  
>>>  	ci = lock_cluster(si, offset);
>>>  	VM_BUG_ON(!cluster_is_huge(ci));
>>> +	VM_BUG_ON(!is_cluster_offset(offset));
>>> +	VM_BUG_ON(cluster_count(ci) < SWAPFILE_CLUSTER);
>>>  	map = si->swap_map + offset;
>>> -	for (i = 0; i < SWAPFILE_CLUSTER; i++) {
>>> -		val = map[i];
>>> -		VM_BUG_ON(!(val & SWAP_HAS_CACHE));
>>> -		if (val == SWAP_HAS_CACHE)
>>> -			free_entries++;
>>> +	if (!cluster_swapcount(ci)) {
>>> +		for (i = 0; i < SWAPFILE_CLUSTER; i++) {
>>> +			val = map[i];
>>> +			VM_BUG_ON(!(val & SWAP_HAS_CACHE));
>>> +			if (val == SWAP_HAS_CACHE)
>>> +				free_entries++;
>>> +		}
>>> +		if (free_entries != SWAPFILE_CLUSTER)
>>> +			cluster_clear_huge(ci);
>>>  	}
>>
>> Also, I'll point out that cluster_swapcount() continues the horrific
>> naming of cluster_couunt(), not saying what the count is *of*.  The
>> return value doesn't help much:
>>
>> 	return cluster_count(ci) - SWAPFILE_CLUSTER;
> 
> We have page_swapcount() for page, swp_swapcount() for swap entry.
> cluster_swapcount() tries to mimic them for swap cluster.  But I am not
> good at naming in general.  What's your suggestion?

I don't have a suggestion because I haven't the foggiest idea what it is
doing. :)

Is it the number of instantiated swap cache pages that are referring to
this cluster?  Is it just huge pages?  Huge and small?  One refcount per
huge page, or 512?