linux-kernel - Re: [PATCH] mm: thp: don't need drain lru cache when splitting and mlocking THP

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Date:   Fri, 3 Apr 2020 09:12:20 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        kirill.shutemov@...ux.intel.com, hughd@...gle.com,
        aarcange@...hat.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: thp: don't need drain lru cache when splitting and
 mlocking THP

On Thu, Apr 02, 2020 at 04:37:06PM -0700, Yang Shi wrote:
> 
> 
> 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@...ux.intel.com>
> > > > Cc: Hugh Dickins <hughd@...gle.com>
> > > > Cc: Andrea Arcangeli <aarcange@...hat.com>
> > > > Signed-off-by: Yang Shi <yang.shi@...ux.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.

Ah, that's right!  The checks in swap.c are for PageCompound.