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

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Date:   Sat, 2 Mar 2019 09:10:43 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Jan Stancek <jstancek@...hat.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        peterz@...radead.org, riel@...riel.com, mhocko@...e.com,
        ying.huang@...el.com, jrdr.linux@...il.com, jglisse@...hat.com,
        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

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.

ie I would drop these lines:

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