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Subject: Re: [PATCHv2 4/8] khugepaged: Drain LRU add pagevec after swapin
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     akpm@linux-foundation.org, Andrea Arcangeli <aarcange@redhat.com>,
        Zi Yan <ziy@nvidia.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
References: <20200403112928.19742-1-kirill.shutemov@linux.intel.com>
 <20200403112928.19742-5-kirill.shutemov@linux.intel.com>
 <620a61ed-be87-f60e-f562-379cd3adbe08@linux.alibaba.com>
 <20200408130532.ktztqf5fabo3x3cd@box>
From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <4b63e949-16bc-f239-89ec-93898cb4d772@linux.alibaba.com>
Date:   Wed, 8 Apr 2020 11:42:55 -0700
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On 4/8/20 6:05 AM, Kirill A. Shutemov wrote:
> On Mon, Apr 06, 2020 at 11:29:11AM -0700, Yang Shi wrote:
>>
>> On 4/3/20 4:29 AM, Kirill A. Shutemov wrote:
>>> __collapse_huge_page_isolate() may fail due to extra pin in the LRU add
>>> pagevec. It's petty common for swapin case: we swap in pages just to
>>> fail due to the extra pin.
>>>
>>> Drain LRU add pagevec on sucessfull swapin.
>>>
>>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>> ---
>>>    mm/khugepaged.c | 5 +++++
>>>    1 file changed, 5 insertions(+)
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index fdc10ffde1ca..57ff287caf6b 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -940,6 +940,11 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
>>>    	}
>>>    	vmf.pte--;
>>>    	pte_unmap(vmf.pte);
>>> +
>>> +	/* Drain LRU add pagevec to remove extra pin on the swapped in pages */
>>> +	if (swapped_in)
>>> +		lru_add_drain();
>> There is already lru_add_drain() called in swap readahead path, please see
>> swap_vma_readahead() and swap_cluster_readahead().
> But not for synchronous case. See SWP_SYNCHRONOUS_IO branch in
> do_swap_page().

Aha, yes. I missed the synchronous case.

>
> Maybe we should drain it in swap_readpage() or in do_swap_page() after
> swap_readpage()? I donno.

It may be better to keep it as is. Draining lru for every page for 
synchronous case in do_swap_page() path sounds not very productive. 
Doing it in khugepaged seems acceptable. We just drain lru cache again 
for non-synchronous case, but the cache may be already empty so it 
should take very short time since nothing to drain.

>