linux-kernel - Re: [RFC PATCH] mm: thp: grab the lock before manipulation defer list

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Message-ID: <alpine.DEB.2.21.2001061803200.55132@chino.kir.corp.google.com>
Date:   Mon, 6 Jan 2020 18:07:29 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Wei Yang <richardw.yang@...ux.intel.com>
cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, vdavydov.dev@...il.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        cgroups@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>
Subject: Re: [RFC PATCH] mm: thp: grab the lock before manipulation defer
 list

On Tue, 7 Jan 2020, Wei Yang wrote:

> >One thing you might want to do is pull the "if (compound)" check out
> >and place it outside of the spinlock check. It would then simplify
> >this signficantly so it is something like
> >if (compound) {
> >  spin_lock();
> >  list = page_deferred_list(page);
> >  if (!list_empty(list)) {
> >    list_del_init(list);
> >    from->..split_queue_len--;
> >  }
> >  spin_unlock();
> >}
> >
> >Same for the block below. I would pull the check for compound outside
> >of the spinlock call since it is a value that shouldn't change and
> >would eliminate an unnecessary lock in the non-compound case.
> 
> This is reasonable, if no objection from others, I would change this in v2.

Looks fine to me; I don't see it as a necessary improvement but there's 
also no reason to object to it.  It's definitely a patch that is needed, 
however, for the simple reason that with the existing code we can 
manipulate the deferred split queue incorrectly so either way works for 
me.  Feel free to keep my acked-by.