Re: [PATCH] mm/mremap_pages: Fix static key devmap_managed_key updates

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 10/23/20 12:40 AM, Ira Weiny wrote:
> On Thu, Oct 22, 2020 at 11:19:43AM -0700, Ralph Campbell wrote:
>>
>> On 10/22/20 8:41 AM, Ira Weiny wrote:
>>> On Thu, Oct 22, 2020 at 11:37:53AM +0530, Aneesh Kumar K.V wrote:
>>>> commit 6f42193fd86e ("memremap: don't use a separate devm action for
>>>> devmap_managed_enable_get") changed the static key updates such that we
>>>> now call devmap_managed_enable_put() without doing the equivalent
>>>> devmap_managed_enable_get().
>>>>
>>>> devmap_managed_enable_get() is only called for MEMORY_DEVICE_PRIVATE and
>>>> MEMORY_DEVICE_FS_DAX, But memunmap_pages() get called for other pgmap
>>>> types too. This results in the below warning when switching between
>>>> system-ram and devdax mode for devdax namespace.
>>>>
>>>>    jump label: negative count!
>>>>    WARNING: CPU: 52 PID: 1335 at kernel/jump_label.c:235 static_key_slow_try_dec+0x88/0xa0
>>>>    Modules linked in:
>>>>    ....
>>>>
>>>>    NIP [c000000000433318] static_key_slow_try_dec+0x88/0xa0
>>>>    LR [c000000000433314] static_key_slow_try_dec+0x84/0xa0
>>>>    Call Trace:
>>>>    [c000000025c1f660] [c000000000433314] static_key_slow_try_dec+0x84/0xa0 (unreliable)
>>>>    [c000000025c1f6d0] [c000000000433664] __static_key_slow_dec_cpuslocked+0x34/0xd0
>>>>    [c000000025c1f700] [c0000000004337a4] static_key_slow_dec+0x54/0xf0
>>>>    [c000000025c1f770] [c00000000059c49c] memunmap_pages+0x36c/0x500
>>>>    [c000000025c1f820] [c000000000d91d10] devm_action_release+0x30/0x50
>>>>    [c000000025c1f840] [c000000000d92e34] release_nodes+0x2f4/0x3e0
>>>>    [c000000025c1f8f0] [c000000000d8b15c] device_release_driver_internal+0x17c/0x280
>>>>    [c000000025c1f930] [c000000000d883a4] bus_remove_device+0x124/0x210
>>>>    [c000000025c1f9b0] [c000000000d80ef4] device_del+0x1d4/0x530
>>>>    [c000000025c1fa70] [c000000000e341e8] unregister_dev_dax+0x48/0xe0
>>>>    [c000000025c1fae0] [c000000000d91d10] devm_action_release+0x30/0x50
>>>>    [c000000025c1fb00] [c000000000d92e34] release_nodes+0x2f4/0x3e0
>>>>    [c000000025c1fbb0] [c000000000d8b15c] device_release_driver_internal+0x17c/0x280
>>>>    [c000000025c1fbf0] [c000000000d87000] unbind_store+0x130/0x170
>>>>    [c000000025c1fc30] [c000000000d862a0] drv_attr_store+0x40/0x60
>>>>    [c000000025c1fc50] [c0000000006d316c] sysfs_kf_write+0x6c/0xb0
>>>>    [c000000025c1fc90] [c0000000006d2328] kernfs_fop_write+0x118/0x280
>>>>    [c000000025c1fce0] [c0000000005a79f8] vfs_write+0xe8/0x2a0
>>>>    [c000000025c1fd30] [c0000000005a7d94] ksys_write+0x84/0x140
>>>>    [c000000025c1fd80] [c00000000003a430] system_call_exception+0x120/0x270
>>>>    [c000000025c1fe20] [c00000000000c540] system_call_common+0xf0/0x27c
>>>>
>>>> Cc: Christoph Hellwig <hch@infradead.org>
>>>> Cc: Dan Williams <dan.j.williams@intel.com>
>>>> Cc: Sachin Sant <sachinp@linux.vnet.ibm.com>
>>>> Cc: linux-nvdimm@lists.01.org
>>>> Cc: Ira Weiny <ira.weiny@intel.com>
>>>> Cc: Jason Gunthorpe <jgg@mellanox.com>
>>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>>>> ---
>>>>    mm/memremap.c | 19 +++++++++++++++----
>>>>    1 file changed, 15 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/mm/memremap.c b/mm/memremap.c
>>>> index 73a206d0f645..d4402ff3e467 100644
>>>> --- a/mm/memremap.c
>>>> +++ b/mm/memremap.c
>>>> @@ -158,6 +158,16 @@ void memunmap_pages(struct dev_pagemap *pgmap)
>>>>    {
>>>>    	unsigned long pfn;
>>>>    	int i;
>>>> +	bool need_devmap_managed = false;
>>>> +
>>>> +	switch (pgmap->type) {
>>>> +	case MEMORY_DEVICE_PRIVATE:
>>>> +	case MEMORY_DEVICE_FS_DAX:
>>>> +		need_devmap_managed = true;
>>>> +		break;
>>>> +	default:
>>>> +		break;
>>>> +	}
>>>
>>> Is it overkill to avoid duplicating this switch logic in
>>> page_is_devmap_managed() by creating another call which can be used here?
>>
>> Perhaps. I can imagine a helper defined in include/linux/mm.h which
>> page_is_devmap_managed() could also call but that would impact a lot of
>> places that include mm.h. Since memremap.c already has to have intimate
>> knowledge of the pgmap->type, I think limiting the change to just what
>> is needed is better for now. So the patch looks OK to me.
>>
>> Looking at this some more, I would suggest changing devmap_managed_enable_get()
>> and devmap_managed_enable_put() to do the special case checking instead of
>> doing it in memremap_pages() and memunmap_pages().
>> Then devmap_managed_enable_get() doesn't need to return an error if
>> CONFIG_DEV_PAGEMAP_OPS isn't defined. I have only compile tested the
>> following.
> 
> This looks cleaner to me.  Aneesh?
> 
> FWIW:
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> 
>

Yes. You can add

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>


-aneesh