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Subject: Re: [PATCH] mm: thp: don't need drain lru cache when splitting and
 mlocking THP
To:     Daniel Jordan <daniel.m.jordan@oracle.com>
Cc:     kirill.shutemov@linux.intel.com, hughd@google.com,
        aarcange@redhat.com, akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <1585337380-97368-1-git-send-email-yang.shi@linux.alibaba.com>
 <20200402230411.7ckwkmd6wwtqfkm2@ca-dmjordan1.us.oracle.com>
 <20200402231731.32imzorzucpj4s6w@ca-dmjordan1.us.oracle.com>
From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <ee712068-cf37-35b8-7dfb-50e290b6b2c4@linux.alibaba.com>
Date:   Thu, 2 Apr 2020 16:37:06 -0700
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On 4/2/20 4:17 PM, Daniel Jordan wrote:
> On Thu, Apr 02, 2020 at 07:04:11PM -0400, Daniel Jordan wrote:
>> On Sat, Mar 28, 2020 at 03:29:40AM +0800, Yang Shi wrote:
>>> Since the commit 8f182270dfec ("mm/swap.c: flush lru pvecs on compound
>>> page arrival") THP would not stay in pagevec anymore.  So the
>>> optimization made by commit d965432234db ("thp: increase
>>> split_huge_page() success rate") doesn't make sense anymore, which tries
>>> to unpin munlocked THPs from pagevec by draining pagevec.
>>>
>>> And draining lru cache before isolating THP in mlock path is unnecessary
>>> either.
>> Can we get some of that nice history in this part too?
>>
>> Draining lru cache before isolating THP in mlock path is also unnecessary.
>> b676b293fb48 ("mm, thp: fix mapped pages avoiding unevictable list on mlock")
>> added it and 9a73f61bdb8a ("thp, mlock: do not mlock PTE-mapped file huge
>> pages") accidentally carried it over after the above optimization went in.
>>
>>> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>> Cc: Hugh Dickins <hughd@google.com>
>>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>> Since we don't mlock pte-mapped THP, it seems these huge pages wouldn't ever be
>> in the pagevecs if I'm understanding it all.
> Actually pte-mapped THP doesn't matter for this, both paths always drain when
> they're working with pmd-mapped THP.

Actually either pte-mapped or pmd-mapped doesn't matter, as long as it 
is compound page the lru cache would be flushed immediately upon the 
arrival of the compound page.