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

[Ira Weiny <ira.weiny@intel.com>]

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?

>  
>  	dev_pagemap_kill(pgmap);
>  	for (i = 0; i < pgmap->nr_range; i++)
> @@ -169,7 +179,8 @@ void memunmap_pages(struct dev_pagemap *pgmap)
>  		pageunmap_range(pgmap, i);
>  
>  	WARN_ONCE(pgmap->altmap.alloc, "failed to free all reserved pages\n");
> -	devmap_managed_enable_put();
> +	if (need_devmap_managed)
> +		devmap_managed_enable_put();
>  }
>  EXPORT_SYMBOL_GPL(memunmap_pages);
>  
> @@ -307,7 +318,7 @@ void *memremap_pages(struct dev_pagemap *pgmap, int nid)
>  		.pgprot = PAGE_KERNEL,
>  	};
>  	const int nr_range = pgmap->nr_range;
> -	bool need_devmap_managed = true;
> +	bool need_devmap_managed = false;

I'm CC'ing Ralph Campbell because I think some of his work has proposed this
same change.

Ira

>  	int error, i;
>  
>  	if (WARN_ONCE(!nr_range, "nr_range must be specified\n"))
> @@ -327,6 +338,7 @@ void *memremap_pages(struct dev_pagemap *pgmap, int nid)
>  			WARN(1, "Missing owner\n");
>  			return ERR_PTR(-EINVAL);
>  		}
> +		need_devmap_managed = true;
>  		break;
>  	case MEMORY_DEVICE_FS_DAX:
>  		if (!IS_ENABLED(CONFIG_ZONE_DEVICE) ||
> @@ -334,13 +346,12 @@ void *memremap_pages(struct dev_pagemap *pgmap, int nid)
>  			WARN(1, "File system DAX not supported\n");
>  			return ERR_PTR(-EINVAL);
>  		}
> +		need_devmap_managed = true;
>  		break;
>  	case MEMORY_DEVICE_GENERIC:
> -		need_devmap_managed = false;
>  		break;
>  	case MEMORY_DEVICE_PCI_P2PDMA:
>  		params.pgprot = pgprot_noncached(params.pgprot);
> -		need_devmap_managed = false;
>  		break;
>  	default:
>  		WARN(1, "Invalid pgmap type %d\n", pgmap->type);
> -- 
> 2.26.2
> 
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