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Date:   Thu, 26 Nov 2020 19:13:15 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC 1/3] mm/hotplug: Pre-validate the address range with
 platform


On 11/25/20 10:34 PM, David Hildenbrand wrote:
> On 23.11.20 03:28, Anshuman Khandual wrote:
>> This introduces memhp_range_allowed() which gets called in various hotplug
>> paths. Then memhp_range_allowed() calls arch_get_addressable_range() via
>> memhp_get_pluggable_range(). This callback can be defined by the platform
>> to provide addressable physical range, depending on whether kernel linear
>> mapping is required or not. This mechanism will prevent platform specific
>> errors deep down during hotplug calls. While here, this drops now redundant
>> check_hotplug_memory_addressable() check in __add_pages().
>>
>> Cc: David Hildenbrand <david@...hat.com>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: linux-mm@...ck.org
>> Cc: linux-kernel@...r.kernel.org
>> Suggested-by: David Hildenbrand <david@...hat.com>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>>  include/linux/memory_hotplug.h | 51 ++++++++++++++++++++++++++++++++++
>>  mm/memory_hotplug.c            | 29 ++++++-------------
>>  mm/memremap.c                  |  9 +++++-
>>  3 files changed, 68 insertions(+), 21 deletions(-)
>>
>> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
>> index 551093b74596..2018c0201672 100644
>> --- a/include/linux/memory_hotplug.h
>> +++ b/include/linux/memory_hotplug.h
>> @@ -6,6 +6,7 @@
>>  #include <linux/spinlock.h>
>>  #include <linux/notifier.h>
>>  #include <linux/bug.h>
>> +#include <linux/range.h>
>>  
>>  struct page;
>>  struct zone;
>> @@ -70,6 +71,56 @@ typedef int __bitwise mhp_t;
>>   */
>>  #define MEMHP_MERGE_RESOURCE	((__force mhp_t)BIT(0))
>>  
>> +/*
>> + * Platforms should define arch_get_addressable_range() which provides
>> + * addressable physical memory range depending upon whether the linear
>> + * mapping is required or not. Returned address range must follow
>> + *
>> + * 1. range.start <= range.end
>> + * 1. Must include end both points i.e range.start and range.end
>> + *
>> + * Nonetheless there is a fallback definition provided here providing
>> + * maximum possible addressable physical range, irrespective of the
>> + * linear mapping requirements.
>> + */
>> +#ifndef arch_get_addressable_range
>> +static inline struct range arch_get_addressable_range(bool need_mapping)
> 
> Why not simply
> 
> "arch_get_mappable_range(void)" (or similar) ?

The current name seems bit better (I guess). Because we are asking for
max addressable range with or without the linear mapping.

> 
> AFAIKs, both implementations (arm64/s390x) simply do the exact same
> thing as memhp_get_pluggable_range() for !need_mapping.

That is for now. Even the range without requiring linear mapping might not
be the same (like now) for every platform as some might have constraints.
So asking the platform ranges with or without linear mapping seems to be
better and could accommodate special cases going forward. Anyways, there
is an always an "all allowing" fallback option nonetheless.

> 
>> +{
>> +	struct range memhp_range = {
>> +		.start = 0UL,
>> +		.end = -1ULL,
>> +	};
>> +	return memhp_range;
>> +}
>> +#endif
>> +
>> +static inline struct range memhp_get_pluggable_range(bool need_mapping)
>> +{
>> +	const u64 max_phys = (1ULL << (MAX_PHYSMEM_BITS + 1)) - 1;
>> +	struct range memhp_range = arch_get_addressable_range(need_mapping);
>> +
>> +	if (memhp_range.start > max_phys) {
>> +		memhp_range.start = 0;
>> +		memhp_range.end = 0;
>> +	}
>> +	memhp_range.end = min_t(u64, memhp_range.end, max_phys);
>> +	return memhp_range;
>> +}
>> +
>> +static inline bool memhp_range_allowed(u64 start, u64 size, bool need_mapping)
>> +{
>> +	struct range memhp_range = memhp_get_pluggable_range(need_mapping);
>> +	u64 end = start + size;
>> +
>> +	if ((start < end) && (start >= memhp_range.start) &&
>> +	   ((end - 1) <= memhp_range.end))
> 
> You can drop quite a lot of () here :)

Will replace with.

if (start < end && start >= memhp_range.start && (end - 1) <= memhp_range.end)

But too much open comparisons looked bit risky initially :)

> 
>> +		return true;
>> +
>> +	WARN(1, "Hotplug memory [%#llx-%#llx] exceeds maximum addressable range [%#llx-%#llx]\n",
>> +		start, end, memhp_range.start, memhp_range.end);
> 
> pr_warn() (or even pr_warn_once())
> 
> while we're at it. No reason to eventually crash a system :)

Didn't quite get it. How could this crash the system ?

> 
>> +	return false;
>> +}
>> +
> 
> 
> I'd suggest moving these functions into mm/memory_hotplug.c and only
> exposing what makes sense. These functions are not on any hot path. You
> can then convert the arch_ function into a "__weak".

Sure, will move them to mm/memory_hotplug.c and change the arch
function into a '__weak'.

> 
>>  /*
>>   * Extended parameters for memory hotplug:
>>   * altmap: alternative allocator for memmap array (optional)
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 63b2e46b6555..9efb6c8558ed 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -284,22 +284,6 @@ static int check_pfn_span(unsigned long pfn, unsigned long nr_pages,
>>  	return 0;
>>  }
>>  
>> -static int check_hotplug_memory_addressable(unsigned long pfn,
>> -					    unsigned long nr_pages)
>> -{
>> -	const u64 max_addr = PFN_PHYS(pfn + nr_pages) - 1;
>> -
>> -	if (max_addr >> MAX_PHYSMEM_BITS) {
>> -		const u64 max_allowed = (1ull << (MAX_PHYSMEM_BITS + 1)) - 1;
>> -		WARN(1,
>> -		     "Hotplugged memory exceeds maximum addressable address, range=%#llx-%#llx, maximum=%#llx\n",
>> -		     (u64)PFN_PHYS(pfn), max_addr, max_allowed);
>> -		return -E2BIG;
>> -	}
>> -
>> -	return 0;
>> -}
>> -
>>  /*
>>   * Reasonably generic function for adding memory.  It is
>>   * expected that archs that support memory hotplug will
>> @@ -317,10 +301,6 @@ int __ref __add_pages(int nid, unsigned long pfn, unsigned long nr_pages,
>>  	if (WARN_ON_ONCE(!params->pgprot.pgprot))
>>  		return -EINVAL;
>>  
>> -	err = check_hotplug_memory_addressable(pfn, nr_pages);
>> -	if (err)
>> -		return err;
>> -
>>  	if (altmap) {
>>  		/*
>>  		 * Validate altmap is within bounds of the total request
>> @@ -1109,6 +1089,9 @@ int __ref __add_memory(int nid, u64 start, u64 size, mhp_t mhp_flags)
>>  	struct resource *res;
>>  	int ret;
>>  
>> +	if (!memhp_range_allowed(start, size, 1))
>> +		return -ERANGE;
> 
> We used to return -E2BIG, no? Maybe better keep that.

ERANGE seems to be better as the range can overrun on either side.

> 
>> +
>>  	res = register_memory_resource(start, size, "System RAM");
>>  	if (IS_ERR(res))
>>  		return PTR_ERR(res);
>> @@ -1123,6 +1106,9 @@ int add_memory(int nid, u64 start, u64 size, mhp_t mhp_flags)
>>  {
>>  	int rc;
>>  
>> +	if (!memhp_range_allowed(start, size, 1))
>> +		return -ERANGE;
>> +
>>  	lock_device_hotplug();
>>  	rc = __add_memory(nid, start, size, mhp_flags);
>>  	unlock_device_hotplug();
>> @@ -1163,6 +1149,9 @@ int add_memory_driver_managed(int nid, u64 start, u64 size,
>>  	    resource_name[strlen(resource_name) - 1] != ')')
>>  		return -EINVAL;
>>  
>> +	if (!memhp_range_allowed(start, size, 0))
>> +		return -ERANGE;
>> +
>>  	lock_device_hotplug();
> 
> For all 3 cases, doing a single check in register_memory_resource() is
> sufficient.

Will replace with a single check in register_memory_resource(). But does
add_memory_driver_managed() always require linear mapping ? The proposed
check here did not ask for linear mapping in add_memory_driver_managed().

> 
>>  
>>  	res = register_memory_resource(start, size, resource_name);
>> diff --git a/mm/memremap.c b/mm/memremap.c
>> index 16b2fb482da1..388a34b068c1 100644
>> --- a/mm/memremap.c
>> +++ b/mm/memremap.c
>> @@ -188,6 +188,7 @@ static int pagemap_range(struct dev_pagemap *pgmap, struct mhp_params *params,
>>  	struct range *range = &pgmap->ranges[range_id];
>>  	struct dev_pagemap *conflict_pgmap;
>>  	int error, is_ram;
>> +	bool is_private = false;
> 
> nit: Reverse christmas tree :)
> 
> 
> const bool is_private = pgmap->type == MEMORY_DEVICE_PRIVATE;

Will replace with this right at the beginning.

> 
>>  
>>  	if (WARN_ONCE(pgmap_altmap(pgmap) && range_id > 0,
>>  				"altmap not supported for multiple ranges\n"))
>> @@ -207,6 +208,9 @@ static int pagemap_range(struct dev_pagemap *pgmap, struct mhp_params *params,
>>  		return -ENOMEM;
>>  	}
>>  
>> +	if (pgmap->type == MEMORY_DEVICE_PRIVATE)
>> +		is_private = true;
>> +
>>  	is_ram = region_intersects(range->start, range_len(range),
>>  		IORESOURCE_SYSTEM_RAM, IORES_DESC_NONE);
>>  
>> @@ -230,6 +234,9 @@ static int pagemap_range(struct dev_pagemap *pgmap, struct mhp_params *params,
>>  	if (error)
>>  		goto err_pfn_remap;
>>  
>> +	if (!memhp_range_allowed(range->start, range_len(range), !is_private))
>> +		goto err_pfn_remap;
>> +
>>  	mem_hotplug_begin();
>>  
>>  	/*
>> @@ -243,7 +250,7 @@ static int pagemap_range(struct dev_pagemap *pgmap, struct mhp_params *params,
>>  	 * the CPU, we do want the linear mapping and thus use
>>  	 * arch_add_memory().
>>  	 */
>> -	if (pgmap->type == MEMORY_DEVICE_PRIVATE) {
>> +	if (is_private) {
>>  		error = add_pages(nid, PHYS_PFN(range->start),
>>  				PHYS_PFN(range_len(range)), params);
>>  	} else {
>>
> 
> Doing these checks in add_pages() / arch_add_memory() would be neater -
> but as they we don't have clean generic wrappers yet, I consider this
> good enough until we have reworked that part. (others might disagree :) )

Sure, will leave the check as is.

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