linux-kernel - Re: [PATCH] mm/memory.c: do_fault: avoid usage of stale vm_area

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Date:   Sat, 2 Mar 2019 13:00:09 -0500 (EST)
From:   Jan Stancek <jstancek@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        peterz@...radead.org, riel@...riel.com, mhocko@...e.com,
        ying huang <ying.huang@...el.com>,
        jrdr linux <jrdr.linux@...il.com>, jglisse@...hat.com,
        aneesh kumar <aneesh.kumar@...ux.ibm.com>, david@...hat.com,
        aarcange@...hat.com, raquini@...hat.com, rientjes@...gle.com,
        kirill@...temov.name, mgorman@...hsingularity.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/memory.c: do_fault: avoid usage of stale
 vm_area_struct



----- 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);
> > +
>