linux-kernel - Re: [External] Re: [PATCH v2 3/6] mm: hugetlb: fix a race between freeing and dissolving the page

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Message-ID: <20210108093136.GY13207@dhcp22.suse.cz>
Date:   Fri, 8 Jan 2021 10:31:36 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.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 v2 3/6] mm: hugetlb: fix a race between
 freeing and dissolving the page

On Fri 08-01-21 17:01:03, Muchun Song wrote:
> On Fri, Jan 8, 2021 at 4:43 PM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Thu 07-01-21 23:11:22, Muchun Song wrote:
[..]
> > > But I find a tricky problem to solve. See free_huge_page().
> > > If we are in non-task context, we should schedule a work
> > > to free the page. We reuse the page->mapping. If the page
> > > is already freed by the dissolve path. We should not touch
> > > the page->mapping. So we need to check PageHuge().
> > > The check and llist_add() should be protected by
> > > hugetlb_lock. But we cannot do that. Right? If dissolve
> > > happens after it is linked to the list. We also should
> > > remove it from the list (hpage_freelist). It seems to make
> > > the thing more complex.
> >
> > I am not sure I follow you here but yes PageHuge under hugetlb_lock
> > should be the reliable way to check for the race. I am not sure why we
> > really need to care about mapping or other state.
> 
> CPU0:                               CPU1:
> free_huge_page(page)
>   if (PageHuge(page))
>                                     dissolve_free_huge_page(page)
>                                       spin_lock(&hugetlb_lock)
>                                       update_and_free_page(page)
>                                       spin_unlock(&hugetlb_lock)
>     llist_add(page->mapping)
>     // the mapping is corrupted
> 
> The PageHuge(page) and llist_add() should be protected by
> hugetlb_lock. Right? If so, we cannot hold hugetlb_lock
> in free_huge_page() path.

OK, I see. I completely forgot about this snowflake. I thought that
free_huge_page was a typo missing initial __. Anyway you are right that
this path needs a check as well. But I don't see why we couldn't use the
lock here. The lock can be held only inside the !in_task branch.
Although it would be much more nicer if the lock was held at this layer
rather than both free_huge_page and __free_huge_page. But that clean up
can be done on top.

-- 
Michal Hocko
SUSE Labs