Re: [PATCH v2] mm: disallow mapping that conflict for devm_memremap_pages()

[Jeff Moyer <jmoyer@redhat.com>]

Hi, Dave,

Dave Jiang <dave.jiang@intel.com> writes:

> When pmem namespaces created are smaller than section size, this can cause
> issue during removal and gpf was observed:
>
> [ 249.613597] general protection fault: 0000 1 SMP PTI
> [ 249.725203] CPU: 36 PID: 3941 Comm: ndctl Tainted: G W
> 4.14.28-1.el7uek.x86_64 #2
> [ 249.745495] task: ffff88acda150000 task.stack: ffffc900233a4000
> [ 249.752107] RIP: 0010:__put_page+0x56/0x79
> [ 249.844675] Call Trace:
> [ 249.847410] devm_memremap_pages_release+0x155/0x23a
> [ 249.852953] release_nodes+0x21e/0x260
> [ 249.857138] devres_release_all+0x3c/0x48
> [ 249.861606] device_release_driver_internal+0x15c/0x207
> [ 249.867439] device_release_driver+0x12/0x14
> [ 249.872204] unbind_store+0xba/0xd8
> [ 249.876098] drv_attr_store+0x27/0x31
> [ 249.880186] sysfs_kf_write+0x3f/0x46
> [ 249.884266] kernfs_fop_write+0x10f/0x18b
> [ 249.888734] __vfs_write+0x3a/0x16d
> [ 249.892628] ? selinux_file_permission+0xe5/0x116
> [ 249.897881] ? security_file_permission+0x41/0xbb
> [ 249.903133] vfs_write+0xb2/0x1a1
> [ 249.906835] ? syscall_trace_enter+0x1ce/0x2b8
> [ 249.911795] SyS_write+0x55/0xb9
> [ 249.915397] do_syscall_64+0x79/0x1ae
> [ 249.919485] entry_SYSCALL_64_after_hwframe+0x3d/0x0
>
> Add code to check whether we have mapping already in the same section and
> prevent additional mapping from created if that is the case.
>
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> ---
>
> v2: Change dev_warn() to dev_WARN() to provide helpful backtrace. (Robert E)

OK, I can reproduce the issue.  What I don't like about your patch is
that you can still get yourself into trouble.  Just create a namespace
with a size that isn't aligned to 128MB, and then all further
create-namespace operations will fail.  The only "fix" is to delete the
odd-sized namespace and try again.  And that warning message doesn't
really help the administrator to figure this out.

Why can't we simply round up to the next section automatically?  Either
that, or have the kernel export a minimum namespace size of 128MB, and
have ndctl enforce it?  I know we had some requests for 4MB namespaces,
but it doesn't sound like those will be very useful if they're going to
waste 124MB of space.

Or, we could try to fix this problem of having multiple namespace
co-exist in the same memblock section.  That seems like the most obvious
fix, but there must be a reason you didn't pursue it.

Dave, what do you think is the most viable option?

Cheers,
Jeff


>  kernel/memremap.c |   18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/memremap.c b/kernel/memremap.c
> index 5857267a4af5..a734b1747466 100644
> --- a/kernel/memremap.c
> +++ b/kernel/memremap.c
> @@ -176,10 +176,27 @@ void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap)
>  	unsigned long pfn, pgoff, order;
>  	pgprot_t pgprot = PAGE_KERNEL;
>  	int error, nid, is_ram;
> +	struct dev_pagemap *conflict_pgmap;
>  
>  	align_start = res->start & ~(SECTION_SIZE - 1);
>  	align_size = ALIGN(res->start + resource_size(res), SECTION_SIZE)
>  		- align_start;
> +	align_end = align_start + align_size - 1;
> +
> +	conflict_pgmap = get_dev_pagemap(PHYS_PFN(align_start), NULL);
> +	if (conflict_pgmap) {
> +		dev_WARN(dev, "Conflicting mapping in same section\n");
> +		put_dev_pagemap(conflict_pgmap);
> +		return ERR_PTR(-ENOMEM);
> +	}
> +
> +	conflict_pgmap = get_dev_pagemap(PHYS_PFN(align_end), NULL);
> +	if (conflict_pgmap) {
> +		dev_WARN(dev, "Conflicting mapping in same section\n");
> +		put_dev_pagemap(conflict_pgmap);
> +		return ERR_PTR(-ENOMEM);
> +	}
> +
>  	is_ram = region_intersects(align_start, align_size,
>  		IORESOURCE_SYSTEM_RAM, IORES_DESC_NONE);
>  
> @@ -199,7 +216,6 @@ void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap)
>  
>  	mutex_lock(&pgmap_lock);
>  	error = 0;
> -	align_end = align_start + align_size - 1;
>  
>  	foreach_order_pgoff(res, order, pgoff) {
>  		error = __radix_tree_insert(&pgmap_radix,
>
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