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Date:   Tue, 12 Jan 2021 17:51:05 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v3 4/6] mm: hugetlb: add return -EAGAIN for dissolve_free_huge_page

On Tue, Jan 12, 2021 at 4:33 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Mon 11-01-21 17:20:51, Mike Kravetz wrote:
> > On 1/10/21 4:40 AM, Muchun Song wrote:
> > > There is a race between dissolve_free_huge_page() and put_page(),
> > > and the race window is quite small. Theoretically, we should return
> > > -EBUSY when we encounter this race. In fact, we have a chance to
> > > successfully dissolve the page if we do a retry. Because the race
> > > window is quite small. If we seize this opportunity, it is an
> > > optimization for increasing the success rate of dissolving page.
> > >
> > > If we free a HugeTLB page from a non-task context, it is deferred
> > > through a workqueue. In this case, we need to flush the work.
> > >
> > > The dissolve_free_huge_page() can be called from memory hotplug,
> > > the caller aims to free the HugeTLB page to the buddy allocator
> > > so that the caller can unplug the page successfully.
> > >
> > > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> > > ---
> > >  mm/hugetlb.c | 26 +++++++++++++++++++++-----
> > >  1 file changed, 21 insertions(+), 5 deletions(-)
> >
> > I am unsure about the need for this patch.  The code is OK, there are no
> > issues with the code.
> >
> > As mentioned in the commit message, this is an optimization and could
> > potentially cause a memory offline operation to succeed instead of fail.
> > However, we are very unlikely to ever exercise this code.  Adding an
> > optimization that is unlikely to be exercised is certainly questionable.
> >
> > Memory offline is the only code that could benefit from this optimization.
> > As someone with more memory offline user experience, what is your opinion
> > Michal?
>
> I am not a great fun of optimizations without any data to back them up.
> I do not see any sign this code has been actually tested and the
> condition triggered.

This race is quite small. I only trigger this only once on my server.
And then the kernel panic. So I sent this patch series to fix some
bugs.

>
> Besides that I have requested to have an explanation of why blocking on
> the WQ is safe and that hasn't happened.

I have seen all the caller of dissolve_free_huge_page, some caller is under
page lock (via lock_page). Others are also under a sleep context.

So I think that blocking on the WQ is safe. Right?

> --
> Michal Hocko
> SUSE Labs

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