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

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Date:   Thu, 2 Apr 2020 16:17:31 -0700 (PDT)
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     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 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.