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Date:   Sat, 2 Mar 2019 13:00:09 -0500 (EST)
From:   Jan Stancek <jstancek@redhat.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     linux-mm@kvack.org, akpm@linux-foundation.org,
        peterz@infradead.org, riel@surriel.com, mhocko@suse.com,
        ying huang <ying.huang@intel.com>,
        jrdr linux <jrdr.linux@gmail.com>, jglisse@redhat.com,
        aneesh kumar <aneesh.kumar@linux.ibm.com>, david@redhat.com,
        aarcange@redhat.com, raquini@redhat.com, rientjes@google.com,
        kirill@shutemov.name, mgorman@techsingularity.net,
        linux-kernel@vger.kernel.org
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References: <0b7a4604529e16ace8d65a42dac7c78582e7fb28.1551538524.git.jstancek@redhat.com> <20190302171043.GP11592@bombadil.infradead.org>
Subject: Re: [PATCH] mm/memory.c: do_fault: avoid usage of stale
 vm_area_struct
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Thread-Topic: mm/memory.c: do_fault: avoid usage of stale vm_area_struct
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----- Original Message -----
> On Sat, Mar 02, 2019 at 04:11:26PM +0100, Jan Stancek wrote:
> > Problem is that "vmf->vma" used in do_fault() can become stale.
> > Because mmap_sem may be released, other threads can come in,
> > call munmap() and cause "vma" be returned to kmem cache, and
> > get zeroed/re-initialized and re-used:
> 
> > This patch pins mm_struct and stores its value, to avoid using
> > potentially stale "vma" when calling pte_free().
> 
> OK, we need to cache the mm_struct, but why do we need the extra atomic op?
> There's surely no way the mm can be freed while the thread is in the middle
> of handling a fault.

You're right, I was needlessly paranoid.

> 
> ie I would drop these lines:

I'll send v2.

Thanks,
Jan

> 
> > +	mmgrab(vm_mm);
> > +
> ...
> > +
> > +	mmdrop(vm_mm);
> > +
>