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Date:   Tue, 12 Jan 2021 19:34:45 +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 7:12 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Tue 12-01-21 18:49:17, Muchun Song wrote:
> > On Tue, Jan 12, 2021 at 6:06 PM Michal Hocko <mhocko@...e.com> wrote:
> > >
> > > On Tue 12-01-21 17:51:05, Muchun Song wrote:
> > > > 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.
> > >
> > > Memory hotplug shouldn't panic when this race happens. Are you sure you
> > > have seen a race that is directly related to this patch?
> >
> > I mean the panic is fixed by:
> >
> >   [PATCH v3 3/6] mm: hugetlb: fix a race between freeing and dissolving the page
>
> OK, so the answer is that this is not really triggered by any real life
> problem. Can you actually trigger it intentionally?
>
> > > > > 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?
> > >
> > > I have requested to explicitly write your thinking why this is safe so
> > > that we can double check it. Dependency on a work queue progress is much
> > > more complex than any other locks because there is no guarantee that WQ
> > > will make forward progress (all workers might be stuck, new workers not
> > > able to be created etc.).
> >
> > OK. I know about your concern. How about setting the page as temporary
> > when hitting this race?
> >
> >  int dissolve_free_huge_page(struct page *page)
> >  {
> > @@ -1793,8 +1794,10 @@ int dissolve_free_huge_page(struct page *page)
> >                  * We should make sure that the page is already on the free list
> >                  * when it is dissolved.
> >                  */
> > -               if (unlikely(!PageHugeFreed(head)))
> > +               if (unlikely(!PageHugeFreed(head))) {
> > +                       SetPageHugeTemporary(page)
> >                         goto out;
> > +               }
> >
> > Setting the page as temporary and just return -EBUSY (do not flush
> > the work). __free_huge_page() will free it to the buddy allocator later.
>
> Can we stop these subtle hacks please? Temporary page is meant to
> represent unaccounted temporary page for migration. This has nothing to
> do with it.

Sure. Can drop this patch.

> --
> Michal Hocko
> SUSE Labs

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