[Xen-devel] [PATCH RFCv2 3/6] mm/memory_hotplug: fix online/offline_pages called w.o. mem_hotplug_lock

[David Hildenbrand <david@xxxxxxxxxx>]

There seem to be some problems as result of 30467e0b3be ("mm, hotplug:
fix concurrent memory hot-add deadlock"), which tried to fix a possible
lock inversion reported and discussed in [1] due to the two locks
        a) device_lock()
        b) mem_hotplug_lock

While add_memory() first takes b), followed by a) during
bus_probe_device(), onlining of memory from user space first took b),
followed by a), exposing a possible deadlock.

In [1], and it was decided to not make use of device_hotplug_lock, but
rather to enforce a locking order.

The problems I spotted related to this:

1. Memory block device attributes: While .state first calls
   mem_hotplug_begin() and the calls device_online() - which takes
   device_lock() - .online does no longer call mem_hotplug_begin(), so
   effectively calls online_pages() without mem_hotplug_lock.

2. device_online() should be called under device_hotplug_lock, however
   onlining memory during add_memory() does not take care of that.

In addition, I think there is also something wrong about the locking in

3. arch/powerpc/platforms/powernv/memtrace.c calls offline_pages()
   without locks. This was introduced after 30467e0b3be. And skimming over
   the code, I assume it could need some more care in regards to locking
   (e.g. device_online() called without device_hotplug_lock - but I'll
   not touch that for now).

Now that we hold the device_hotplug_lock when
- adding memory (e.g. via add_memory()/add_memory_resource())
- removing memory (e.g. via remove_memory())
- device_online()/device_offline()

We can move mem_hotplug_lock usage back into
online_pages()/offline_pages().

Why is mem_hotplug_lock still needed? Essentially to make
get_online_mems()/put_online_mems() be very fast (relying on
device_hotplug_lock would be very slow), and to serialize against
addition of memory that does not create memory block devices (hmm).

[1] http://driverdev.linuxdriverproject.org/pipermail/ driverdev-devel/
    2015-February/065324.html

This patch is partly based on a patch by Vitaly Kuznetsov.

Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
Cc: Len Brown <lenb@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
Cc: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
Cc: Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Rashmica Gupta <rashmica.g@xxxxxxxxx>
Cc: Michael Neuling <mikey@xxxxxxxxxxx>
Cc: Balbir Singh <bsingharora@xxxxxxxxx>
Cc: Kate Stewart <kstewart@xxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Philippe Ombredanne <pombredanne@xxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: YASUAKI ISHIMATSU <yasu.isimatu@xxxxxxxxx>
Cc: Mathieu Malaterre <malat@xxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 drivers/base/memory.c | 13 +------------
 mm/memory_hotplug.c   | 28 ++++++++++++++++++++--------
 2 files changed, 21 insertions(+), 20 deletions(-)

diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index 5b0375be7f65..04be13539eb8 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -228,7 +228,6 @@ static bool pages_correctly_probed(unsigned long start_pfn)
 /*
  * MEMORY_HOTPLUG depends on SPARSEMEM in mm/Kconfig, so it is
  * OK to have direct references to sparsemem variables in here.
- * Must already be protected by mem_hotplug_begin().
  */
 static int
 memory_block_action(unsigned long phys_index, unsigned long action, int 
online_type)
@@ -294,7 +293,6 @@ static int memory_subsys_online(struct device *dev)
        if (mem->online_type < 0)
                mem->online_type = MMOP_ONLINE_KEEP;
 
-       /* Already under protection of mem_hotplug_begin() */
        ret = memory_block_change_state(mem, MEM_ONLINE, MEM_OFFLINE);
 
        /* clear online_type */
@@ -341,19 +339,11 @@ store_mem_state(struct device *dev,
                goto err;
        }
 
-       /*
-        * Memory hotplug needs to hold mem_hotplug_begin() for probe to find
-        * the correct memory block to online before doing device_online(dev),
-        * which will take dev->mutex.  Take the lock early to prevent an
-        * inversion, memory_subsys_online() callbacks will be implemented by
-        * assuming it's already protected.
-        */
-       mem_hotplug_begin();
-
        switch (online_type) {
        case MMOP_ONLINE_KERNEL:
        case MMOP_ONLINE_MOVABLE:
        case MMOP_ONLINE_KEEP:
+               /* mem->online_type is protected by device_hotplug_lock */
                mem->online_type = online_type;
                ret = device_online(&mem->dev);
                break;
@@ -364,7 +354,6 @@ store_mem_state(struct device *dev,
                ret = -EINVAL; /* should never happen */
        }
 
-       mem_hotplug_done();
 err:
        unlock_device_hotplug();
 
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index e2b5c751e3ea..a2c6c87d83f3 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -881,7 +881,6 @@ static struct zone * __meminit move_pfn_range(int 
online_type, int nid,
        return zone;
 }
 
-/* Must be protected by mem_hotplug_begin() or a device_lock */
 int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int 
online_type)
 {
        unsigned long flags;
@@ -893,6 +892,8 @@ int __ref online_pages(unsigned long pfn, unsigned long 
nr_pages, int online_typ
        struct memory_notify arg;
        struct memory_block *mem;
 
+       mem_hotplug_begin();
+
        /*
         * We can't use pfn_to_nid() because nid might be stored in struct page
         * which is not yet initialized. Instead, we find nid from memory block.
@@ -957,6 +958,7 @@ int __ref online_pages(unsigned long pfn, unsigned long 
nr_pages, int online_typ
 
        if (onlined_pages)
                memory_notify(MEM_ONLINE, &arg);
+       mem_hotplug_done();
        return 0;
 
 failed_addition:
@@ -964,6 +966,7 @@ int __ref online_pages(unsigned long pfn, unsigned long 
nr_pages, int online_typ
                 (unsigned long long) pfn << PAGE_SHIFT,
                 (((unsigned long long) pfn + nr_pages) << PAGE_SHIFT) - 1);
        memory_notify(MEM_CANCEL_ONLINE, &arg);
+       mem_hotplug_done();
        return ret;
 }
 #endif /* CONFIG_MEMORY_HOTPLUG_SPARSE */
@@ -1168,20 +1171,20 @@ int __ref add_memory_resource(int nid, struct resource 
*res, bool online)
        /* create new memmap entry */
        firmware_map_add_hotplug(start, start + size, "System RAM");
 
+       /* device_online() will take the lock when calling online_pages() */
+       mem_hotplug_done();
+
        /* online pages if requested */
        if (online)
                walk_memory_range(PFN_DOWN(start), PFN_UP(start + size - 1),
                                  NULL, online_memory_block);
 
-       goto out;
-
+       return ret;
 error:
        /* rollback pgdat allocation and others */
        if (new_node)
                rollback_node_hotadd(nid);
        memblock_remove(start, size);
-
-out:
        mem_hotplug_done();
        return ret;
 }
@@ -1621,10 +1624,16 @@ static int __ref __offline_pages(unsigned long 
start_pfn,
                return -EINVAL;
        if (!IS_ALIGNED(end_pfn, pageblock_nr_pages))
                return -EINVAL;
+
+       mem_hotplug_begin();
+
        /* This makes hotplug much easier...and readable.
           we assume this for now. .*/
-       if (!test_pages_in_a_zone(start_pfn, end_pfn, &valid_start, &valid_end))
+       if (!test_pages_in_a_zone(start_pfn, end_pfn, &valid_start,
+                                 &valid_end)) {
+               mem_hotplug_done();
                return -EINVAL;
+       }
 
        zone = page_zone(pfn_to_page(valid_start));
        node = zone_to_nid(zone);
@@ -1633,8 +1642,10 @@ static int __ref __offline_pages(unsigned long start_pfn,
        /* set above range as isolated */
        ret = start_isolate_page_range(start_pfn, end_pfn,
                                       MIGRATE_MOVABLE, true);
-       if (ret)
+       if (ret) {
+               mem_hotplug_done();
                return ret;
+       }
 
        arg.start_pfn = start_pfn;
        arg.nr_pages = nr_pages;
@@ -1705,6 +1716,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
        writeback_set_ratelimit();
 
        memory_notify(MEM_OFFLINE, &arg);
+       mem_hotplug_done();
        return 0;
 
 failed_removal:
@@ -1714,10 +1726,10 @@ static int __ref __offline_pages(unsigned long 
start_pfn,
        memory_notify(MEM_CANCEL_OFFLINE, &arg);
        /* pushback to free area */
        undo_isolate_page_range(start_pfn, end_pfn, MIGRATE_MOVABLE);
+       mem_hotplug_done();
        return ret;
 }
 
-/* Must be protected by mem_hotplug_begin() or a device_lock */
 int offline_pages(unsigned long start_pfn, unsigned long nr_pages)
 {
        return __offline_pages(start_pfn, start_pfn + nr_pages);
-- 
2.17.1


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