linux-kernel - Re: [PATCH v14 07/20] mm/thp: narrow lru locking

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Date:   Thu, 9 Jul 2020 18:48:16 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Johannes Weiner <hannes@...xchg.org>,
        akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        tj@...nel.org, khlebnikov@...dex-team.ru,
        daniel.m.jordan@...cle.com, yang.shi@...ux.alibaba.com,
        lkp@...el.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, shakeelb@...gle.com,
        iamjoonsoo.kim@....com, richard.weiyang@...il.com
Subject: Re: [PATCH v14 07/20] mm/thp: narrow lru locking

On Mon, Jul 06, 2020 at 09:52:34PM -0700, Hugh Dickins wrote:
> On Mon, 6 Jul 2020, Matthew Wilcox wrote:
> > On Mon, Jul 06, 2020 at 05:15:09PM +0800, Alex Shi wrote:
> > > Hi Kirill & Johannes & Matthew,
> 
> Adding Kirill, who was in patch's Cc list but not mail's Cc list.
> 
> I asked Alex to direct this one particularly to Kirill and Johannes
> and Matthew because (and I regret that the commit message still does
> not make this at all clear) this patch changes the lock ordering:
> which for years has been lru_lock outside memcg move_lock outside
> i_pages lock, but here inverted to lru_lock inside i_pages lock.
> 
> I don't see a strong reason to have them one way round or the other,
> and think Alex is right that they can safely be reversed here: but
> he doesn't actually give any reason for doing so (if cleanup, then
> I think the cleanup should have been taken further), and no reason
> for doing so as part of this series.

I've looked around and changing order of lru_lock wrt. i_pages lock seems
safe. I don't have much experience with memcg move_lock.

Alex, if you are going ahead with the patch, please document the locking
order. We have some locking orders listed at the beginning of filemap.c
and rmap.c.

local_irq_disable() also deserves a comment.

-- 
 Kirill A. Shutemov