linux-kernel - Re: [PATCH v3 4/6] mm: hugetlb: add return -EAGAIN for dissolve_free_huge

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Message-ID: <20210112083335.GH22493@dhcp22.suse.cz>
Date:   Tue, 12 Jan 2021 09:33:35 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Muchun Song <songmuchun@...edance.com>, akpm@...ux-foundation.org,
        n-horiguchi@...jp.nec.com, ak@...ux.intel.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/6] mm: hugetlb: add return -EAGAIN for
 dissolve_free_huge_page

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.

Besides that I have requested to have an explanation of why blocking on
the WQ is safe and that hasn't happened.
-- 
Michal Hocko
SUSE Labs