[patch] mm: mlock: document scary-looking stack expansion mlock chain

[patch] mm: mlock: document scary-looking stack expansion mlock chain

[Johannes Weiner]

The fact that mlock calls get_user_pages, and get_user_pages might
call mlock when expanding a stack looks like a potential recursion.

However, mlock makes sure the requested range is already contained
within a vma, so no stack expansion will actually happen from mlock.

Should this ever change: the stack expansion mlocks only the newly
expanded range and so will not result in recursive expansion.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/mlock.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/mlock.c b/mm/mlock.c
index b1647fb..78c4924 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -185,6 +185,10 @@ long __mlock_vma_pages_range(struct vm_area_struct *vma,
 	if (vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC))
 		gup_flags |= FOLL_FORCE;
 
+	/*
+	 * We made sure addr is within a VMA, so the following will
+	 * not result in a stack expansion that recurses back here.
+	 */
 	return __get_user_pages(current, mm, addr, nr_pages, gup_flags,
 				NULL, NULL, nonblocking);
 }
-- 
1.7.11.7

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Re: [patch] mm: mlock: document scary-looking stack expansion mlock chain

[Ric Mason]

On 02/01/2013 02:10 PM, Johannes Weiner wrote:
> The fact that mlock calls get_user_pages, and get_user_pages might
> call mlock when expanding a stack looks like a potential recursion.

Why expand stack need call mlock? I can't find it in the codes, could 
you point out to me?

>
> However, mlock makes sure the requested range is already contained
> within a vma, so no stack expansion will actually happen from mlock.
>
> Should this ever change: the stack expansion mlocks only the newly
> expanded range and so will not result in recursive expansion.
>
> Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>   mm/mlock.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/mm/mlock.c b/mm/mlock.c
> index b1647fb..78c4924 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -185,6 +185,10 @@ long __mlock_vma_pages_range(struct vm_area_struct *vma,
>   	if (vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC))
>   		gup_flags |= FOLL_FORCE;
>   
> +	/*
> +	 * We made sure addr is within a VMA, so the following will
> +	 * not result in a stack expansion that recurses back here.
> +	 */
>   	return __get_user_pages(current, mm, addr, nr_pages, gup_flags,
>   				NULL, NULL, nonblocking);
>   }

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Re: [patch] mm: mlock: document scary-looking stack expansion mlock chain

[Michel Lespinasse]

On Wed, Feb 20, 2013 at 8:51 PM, Ric Mason <ric.masonn@gmail.com> wrote:
> On 02/01/2013 02:10 PM, Johannes Weiner wrote:
>>
>> The fact that mlock calls get_user_pages, and get_user_pages might
>> call mlock when expanding a stack looks like a potential recursion.
>
> Why expand stack need call mlock? I can't find it in the codes, could you
> point out to me?

Its hidden in find_expand_vma(). Basically if the existing stack is
already mlocked, any additional stack expansions get mlocked as well.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.

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Re: [patch] mm: mlock: document scary-looking stack expansion mlock chain

[Michel Lespinasse]

On Fri, Feb 1, 2013 at 2:10 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> The fact that mlock calls get_user_pages, and get_user_pages might
> call mlock when expanding a stack looks like a potential recursion.
>
> However, mlock makes sure the requested range is already contained
> within a vma, so no stack expansion will actually happen from mlock.
>
> Should this ever change: the stack expansion mlocks only the newly
> expanded range and so will not result in recursive expansion.
>
> Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Michel Lespinasse <walken@google.com>

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.

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