[PATCH 1/2] mm/page_alloc: avoid potential NULL pointer dereference

[PATCH 1/2] mm/page_alloc: avoid potential NULL pointer dereference

[Andrey Ryabinin]

ac.preferred_zoneref->zone passed to alloc_flags_nofragment() can be NULL.
'zone' pointer unconditionally derefernced in alloc_flags_nofragment().
Bail out on NULL zone to avoid potential crash.
Currently we don't see any crashes only because alloc_flags_nofragment()
has another bug which allows compiler to optimize away all accesses to
'zone'.

Fixes: 6bb154504f8b ("mm, page_alloc: spread allocations across zones before introducing fragmentation")
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
---
 mm/page_alloc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 933bd42899e8..2b2c7065102f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3461,6 +3461,9 @@ alloc_flags_nofragment(struct zone *zone, gfp_t gfp_mask)
 		alloc_flags |= ALLOC_KSWAPD;
 
 #ifdef CONFIG_ZONE_DMA32
+	if (!zone)
+		return alloc_flags;
+
 	if (zone_idx(zone) != ZONE_NORMAL)
 		goto out;
 
-- 
2.21.0

[PATCH 2/2] mm/page_alloc: fix never set ALLOC_NOFRAGMENT flag

[Andrey Ryabinin]

Commit 0a79cdad5eb2 ("mm: use alloc_flags to record if kswapd can wake")
removed setting of the ALLOC_NOFRAGMENT flag. Bring it back.

Fixes: 0a79cdad5eb2 ("mm: use alloc_flags to record if kswapd can wake")
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
---
 mm/page_alloc.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2b2c7065102f..a85b8252c5ad 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3465,7 +3465,7 @@ alloc_flags_nofragment(struct zone *zone, gfp_t gfp_mask)
 		return alloc_flags;
 
 	if (zone_idx(zone) != ZONE_NORMAL)
-		goto out;
+		return alloc_flags;
 
 	/*
 	 * If ZONE_DMA32 exists, assume it is the one after ZONE_NORMAL and
@@ -3474,9 +3474,9 @@ alloc_flags_nofragment(struct zone *zone, gfp_t gfp_mask)
 	 */
 	BUILD_BUG_ON(ZONE_NORMAL - ZONE_DMA32 != 1);
 	if (nr_online_nodes > 1 && !populated_zone(--zone))
-		goto out;
+		return alloc_flags;
 
-out:
+	alloc_flags |= ALLOC_NOFRAGMENT;
 #endif /* CONFIG_ZONE_DMA32 */
 	return alloc_flags;
 }
-- 
2.21.0

Re: [PATCH 2/2] mm/page_alloc: fix never set ALLOC_NOFRAGMENT flag

[Mel Gorman]

On Tue, Apr 23, 2019 at 03:08:06PM +0300, Andrey Ryabinin wrote:
> Commit 0a79cdad5eb2 ("mm: use alloc_flags to record if kswapd can wake")
> removed setting of the ALLOC_NOFRAGMENT flag. Bring it back.
> 
> Fixes: 0a79cdad5eb2 ("mm: use alloc_flags to record if kswapd can wake")
> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs

Re: [PATCH 1/2] mm/page_alloc: avoid potential NULL pointer dereference

[Mel Gorman]

On Tue, Apr 23, 2019 at 03:08:05PM +0300, Andrey Ryabinin wrote:
> ac.preferred_zoneref->zone passed to alloc_flags_nofragment() can be NULL.
> 'zone' pointer unconditionally derefernced in alloc_flags_nofragment().
> Bail out on NULL zone to avoid potential crash.
> Currently we don't see any crashes only because alloc_flags_nofragment()
> has another bug which allows compiler to optimize away all accesses to
> 'zone'.
> 
> Fixes: 6bb154504f8b ("mm, page_alloc: spread allocations across zones before introducing fragmentation")
> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs

Re: [PATCH 2/2] mm/page_alloc: fix never set ALLOC_NOFRAGMENT flag

[Andrew Morton]

On Tue, 23 Apr 2019 15:08:06 +0300 Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:

> Commit 0a79cdad5eb2 ("mm: use alloc_flags to record if kswapd can wake")
> removed setting of the ALLOC_NOFRAGMENT flag. Bring it back.

What are the runtime effects of this fix?

Re: [PATCH 2/2] mm/page_alloc: fix never set ALLOC_NOFRAGMENT flag

[Mel Gorman]

On Tue, Apr 23, 2019 at 12:01:43PM -0700, Andrew Morton wrote:
> On Tue, 23 Apr 2019 15:08:06 +0300 Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> 
> > Commit 0a79cdad5eb2 ("mm: use alloc_flags to record if kswapd can wake")
> > removed setting of the ALLOC_NOFRAGMENT flag. Bring it back.
> 
> What are the runtime effects of this fix?

The runtime effect is that ALLOC_NOFRAGMENT behaviour is restored so
that allocations are spread across local zones to avoid fragmentation
due to mixing pageblocks as long as possible.

-- 
Mel Gorman
SUSE Labs

Re: [PATCH 2/2] mm/page_alloc: fix never set ALLOC_NOFRAGMENT flag

[Andrew Morton]

On Wed, 24 Apr 2019 10:04:03 +0100 Mel Gorman <mgorman@techsingularity.net> wrote:

> On Tue, Apr 23, 2019 at 12:01:43PM -0700, Andrew Morton wrote:
> > On Tue, 23 Apr 2019 15:08:06 +0300 Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> > 
> > > Commit 0a79cdad5eb2 ("mm: use alloc_flags to record if kswapd can wake")
> > > removed setting of the ALLOC_NOFRAGMENT flag. Bring it back.
> > 
> > What are the runtime effects of this fix?
> 
> The runtime effect is that ALLOC_NOFRAGMENT behaviour is restored so
> that allocations are spread across local zones to avoid fragmentation
> due to mixing pageblocks as long as possible.

OK, thanks.  Is this worth a -stable backport?

Re: [PATCH 2/2] mm/page_alloc: fix never set ALLOC_NOFRAGMENT flag

[Mel Gorman]

On Wed, Apr 24, 2019 at 03:46:24PM -0700, Andrew Morton wrote:
> On Wed, 24 Apr 2019 10:04:03 +0100 Mel Gorman <mgorman@techsingularity.net> wrote:
> 
> > On Tue, Apr 23, 2019 at 12:01:43PM -0700, Andrew Morton wrote:
> > > On Tue, 23 Apr 2019 15:08:06 +0300 Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> > > 
> > > > Commit 0a79cdad5eb2 ("mm: use alloc_flags to record if kswapd can wake")
> > > > removed setting of the ALLOC_NOFRAGMENT flag. Bring it back.
> > > 
> > > What are the runtime effects of this fix?
> > 
> > The runtime effect is that ALLOC_NOFRAGMENT behaviour is restored so
> > that allocations are spread across local zones to avoid fragmentation
> > due to mixing pageblocks as long as possible.
> 
> OK, thanks.  Is this worth a -stable backport?

Yes, but only for 5.0 obviously and both should be included if that is
the case. I did not push for it initially as problems in this area are
hard for a general user to detect and people have not complained about
5.0's fragmentation handling.

-- 
Mel Gorman
SUSE Labs

Re: [PATCH 2/2] mm/page_alloc: fix never set ALLOC_NOFRAGMENT flag

[Andrew Morton]

On Thu, 25 Apr 2019 00:40:53 +0100 Mel Gorman <mgorman@techsingularity.net> wrote:

> On Wed, Apr 24, 2019 at 03:46:24PM -0700, Andrew Morton wrote:
> > On Wed, 24 Apr 2019 10:04:03 +0100 Mel Gorman <mgorman@techsingularity.net> wrote:
> > 
> > > On Tue, Apr 23, 2019 at 12:01:43PM -0700, Andrew Morton wrote:
> > > > On Tue, 23 Apr 2019 15:08:06 +0300 Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> > > > 
> > > > > Commit 0a79cdad5eb2 ("mm: use alloc_flags to record if kswapd can wake")
> > > > > removed setting of the ALLOC_NOFRAGMENT flag. Bring it back.
> > > > 
> > > > What are the runtime effects of this fix?
> > > 
> > > The runtime effect is that ALLOC_NOFRAGMENT behaviour is restored so
> > > that allocations are spread across local zones to avoid fragmentation
> > > due to mixing pageblocks as long as possible.
> > 
> > OK, thanks.  Is this worth a -stable backport?
> 
> Yes, but only for 5.0 obviously and both should be included if that is
> the case. I did not push for it initially as problems in this area are
> hard for a general user to detect and people have not complained about
> 5.0's fragmentation handling.

Ah, OK.  0a79cdad5eb2 didn't have a -stable tag so I suppose we can
leave this patch un-stabled.

If they went and backported 0a79cdad5eb2 anyway, let's hope the scripts
are smart enough to catch this patch's Fixes: link.