[PATCH] mm: don't overwrite mm->def_flags in do_mlockall()

[PATCH] mm: don't overwrite mm->def_flags in do_mlockall()

[Gerald Schaefer]

With commit 8e72033 "thp: make MADV_HUGEPAGE check for mm->def_flags"
the VM_NOHUGEPAGE flag may be set on s390 in mm->def_flags for certain
processes, to prevent future thp mappings. This would be overwritten
by do_mlockall(), which sets it back to 0 with an optional VM_LOCKED
flag set.

To fix this, instead of overwriting mm->def_flags in do_mlockall(),
only the VM_LOCKED flag should be set or cleared.

Reported-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
---
 mm/mlock.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/mlock.c b/mm/mlock.c
index f0b9ce5..c9bd528 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -517,11 +517,11 @@ SYSCALL_DEFINE2(munlock, unsigned long, start, size_t, len)
 static int do_mlockall(int flags)
 {
 	struct vm_area_struct * vma, * prev = NULL;
-	unsigned int def_flags = 0;
 
 	if (flags & MCL_FUTURE)
-		def_flags = VM_LOCKED;
-	current->mm->def_flags = def_flags;
+		current->mm->def_flags |= VM_LOCKED;
+	else
+		current->mm->def_flags &= ~VM_LOCKED;
 	if (flags == MCL_FUTURE)
 		goto out;
 
-- 
1.7.12.4

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH] mm: don't overwrite mm->def_flags in do_mlockall()

[Andrew Morton]

On Wed,  6 Feb 2013 16:49:34 +0100
Gerald Schaefer <gerald.schaefer@de.ibm.com> wrote:

> With commit 8e72033 "thp: make MADV_HUGEPAGE check for mm->def_flags"
> the VM_NOHUGEPAGE flag may be set on s390 in mm->def_flags for certain
> processes, to prevent future thp mappings. This would be overwritten
> by do_mlockall(), which sets it back to 0 with an optional VM_LOCKED
> flag set.
> 
> To fix this, instead of overwriting mm->def_flags in do_mlockall(),
> only the VM_LOCKED flag should be set or cleared.

What are the user-visible effects here?  Looking at the 274023da1e8
changelog, I'm guessing that it might be pretty nasty - kvm breakage?

> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -517,11 +517,11 @@ SYSCALL_DEFINE2(munlock, unsigned long, start, size_t, len)
>  static int do_mlockall(int flags)
>  {
>  	struct vm_area_struct * vma, * prev = NULL;
> -	unsigned int def_flags = 0;
>  
>  	if (flags & MCL_FUTURE)
> -		def_flags = VM_LOCKED;
> -	current->mm->def_flags = def_flags;
> +		current->mm->def_flags |= VM_LOCKED;
> +	else
> +		current->mm->def_flags &= ~VM_LOCKED;
>  	if (flags == MCL_FUTURE)
>  		goto out;

Michal sent an equivalent patch last month:
http://ozlabs.org/~akpm/mmotm/broken-out/mm-make-mlockall-preserve-flags-other-than-vm_locked-in-def_flags.patch.


--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH] mm: don't overwrite mm->def_flags in do_mlockall()

[Gerald Schaefer]

On Wed, 6 Feb 2013 12:51:03 -0800
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Wed,  6 Feb 2013 16:49:34 +0100
> Gerald Schaefer <gerald.schaefer@de.ibm.com> wrote:
> 
> > With commit 8e72033 "thp: make MADV_HUGEPAGE check for
> > mm->def_flags" the VM_NOHUGEPAGE flag may be set on s390 in
> > mm->def_flags for certain processes, to prevent future thp
> > mappings. This would be overwritten by do_mlockall(), which sets it
> > back to 0 with an optional VM_LOCKED flag set.
> > 
> > To fix this, instead of overwriting mm->def_flags in do_mlockall(),
> > only the VM_LOCKED flag should be set or cleared.
> 
> What are the user-visible effects here?  Looking at the 274023da1e8
> changelog, I'm guessing that it might be pretty nasty - kvm breakage?

Yes, though at the moment there should be no mlockall()/munlockall()
involved with kvm/qemu. So currently no user-visible effects, Vivek
found this while reading the do_mlockall() code, but it might be a
good idea to add this to stable.
Could you add a "Cc: stable@vger.kernel.org # v3.7+"?

Thanks,
Gerald

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>