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Subject: Re: [PATCH v3] kernel/resource: Fix locking in
 request_free_mem_region
To:     Alistair Popple <apopple@nvidia.com>, <akpm@linux-foundation.org>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        <david@redhat.com>, <daniel.vetter@ffwll.ch>,
        <dan.j.williams@intel.com>, <gregkh@linuxfoundation.org>,
        <jglisse@redhat.com>, <bsingharora@gmail.com>,
        kernel test robot <oliver.sang@intel.com>
References: <20210330003842.18948-1-apopple@nvidia.com>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <801cd23e-9ac0-adb6-88ba-63ad18d4664c@nvidia.com>
Date:   Mon, 29 Mar 2021 20:42:34 -0700
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On 3/29/21 5:38 PM, Alistair Popple wrote:
> request_free_mem_region() is used to find an empty range of physical
> addresses for hotplugging ZONE_DEVICE memory. It does this by iterating
> over the range of possible addresses using region_intersects() to see if
> the range is free.
> 
> region_intersects() obtains a read lock before walking the resource tree
> to protect against concurrent changes. However it drops the lock prior
> to returning. This means by the time request_mem_region() is called in
> request_free_mem_region() another thread may have already reserved the
> requested region resulting in unexpected failures and a message in the
> kernel log from hitting this condition:
> 
>          /*
>           * mm/hmm.c reserves physical addresses which then
>           * become unavailable to other users.  Conflicts are
>           * not expected.  Warn to aid debugging if encountered.
>           */
>          if (conflict->desc == IORES_DESC_DEVICE_PRIVATE_MEMORY) {
>                  pr_warn("Unaddressable device %s %pR conflicts with %pR",
>                          conflict->name, conflict, res);
> 
> To fix this create versions of region_intersects() and
> request_mem_region() that allow the caller to take the appropriate lock
> such that it may be held over the required calls.
> 
> Instead of creating another version of devm_request_mem_region() that
> doesn't take the lock open-code it to allow the caller to pre-allocate
> the required memory prior to taking the lock.
> 
> Fixes: 0c385190392d8 ("resource: add a not device managed request_free_mem_region variant")
> Fixes: 0092908d16c60 ("mm: factor out a devm_request_free_mem_region helper")

Hi Alistair!

The above "Fixes:" tag looks wrong to me, because that commit did not create
the broken locking that this patch fixes. Therefore, I think that particular line
should be removed from the commit description.

Another note below:

> Fixes: 4ef589dc9b10c ("mm/hmm/devmem: device memory hotplug using ZONE_DEVICE")
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> Acked-by: Balbir Singh <bsingharora@gmail.com>
> Reported-by: kernel test robot <oliver.sang@intel.com>
> 
> ---
> 
> Hi Andrew,
> 
> This fixes a boot issue reported by the kernel test robot with the
> previous version of the patch on x86 with CONFIG_IO_STRICT_DEVMEM=y.
> This was due to the platform specific implementation of
> devmem_is_allowed() creating a recursive lock which I missed. I notice
> you have put v2 in mmtom so apologies for the churn but can you please
> use this version instead? Thanks.
> 
>   - Alistair
> ---
>   kernel/resource.c | 142 ++++++++++++++++++++++++++++++----------------
>   1 file changed, 92 insertions(+), 50 deletions(-)
> 
> diff --git a/kernel/resource.c b/kernel/resource.c
> index 627e61b0c124..7061b9f903ca 100644
> --- a/kernel/resource.c
> +++ b/kernel/resource.c
> @@ -523,6 +523,34 @@ int __weak page_is_ram(unsigned long pfn)
>   }
>   EXPORT_SYMBOL_GPL(page_is_ram);
>   
> +static int __region_intersects(resource_size_t start, size_t size,
> +			       unsigned long flags, unsigned long desc)
> +{
> +	struct resource res;
> +	int type = 0; int other = 0;
> +	struct resource *p;
> +
> +	res.start = start;
> +	res.end = start + size - 1;
> +
> +	for (p = iomem_resource.child; p ; p = p->sibling) {
> +		bool is_type = (((p->flags & flags) == flags) &&
> +				((desc == IORES_DESC_NONE) ||
> +				 (desc == p->desc)));
> +
> +		if (resource_overlaps(p, &res))
> +			is_type ? type++ : other++;
> +	}
> +
> +	if (type == 0)
> +		return REGION_DISJOINT;
> +
> +	if (other == 0)
> +		return REGION_INTERSECTS;
> +
> +	return REGION_MIXED;
> +}
> +
>   /**
>    * region_intersects() - determine intersection of region with known resources
>    * @start: region start address
> @@ -546,31 +574,12 @@ EXPORT_SYMBOL_GPL(page_is_ram);
>   int region_intersects(resource_size_t start, size_t size, unsigned long flags,
>   		      unsigned long desc)
>   {
> -	struct resource res;
> -	int type = 0; int other = 0;
> -	struct resource *p;
> -
> -	res.start = start;
> -	res.end = start + size - 1;
> +	int rc;
>   
>   	read_lock(&resource_lock);
> -	for (p = iomem_resource.child; p ; p = p->sibling) {
> -		bool is_type = (((p->flags & flags) == flags) &&
> -				((desc == IORES_DESC_NONE) ||
> -				 (desc == p->desc)));
> -
> -		if (resource_overlaps(p, &res))
> -			is_type ? type++ : other++;
> -	}
> +	rc = __region_intersects(start, size, flags, desc);
>   	read_unlock(&resource_lock);
> -
> -	if (type == 0)
> -		return REGION_DISJOINT;
> -
> -	if (other == 0)
> -		return REGION_INTERSECTS;
> -
> -	return REGION_MIXED;
> +	return rc;
>   }
>   EXPORT_SYMBOL_GPL(region_intersects);
>   
> @@ -1171,31 +1180,16 @@ struct address_space *iomem_get_mapping(void)
>   	return smp_load_acquire(&iomem_inode)->i_mapping;
>   }
>   
> -/**
> - * __request_region - create a new busy resource region
> - * @parent: parent resource descriptor
> - * @start: resource start address
> - * @n: resource region size
> - * @name: reserving caller's ID string
> - * @flags: IO resource flags
> - */
> -struct resource * __request_region(struct resource *parent,
> -				   resource_size_t start, resource_size_t n,
> -				   const char *name, int flags)
> +static bool request_region_locked(struct resource *parent,
> +				    struct resource *res, resource_size_t start,
> +				    resource_size_t n, const char *name, int flags)
>   {
>   	DECLARE_WAITQUEUE(wait, current);
> -	struct resource *res = alloc_resource(GFP_KERNEL);
> -	struct resource *orig_parent = parent;
> -
> -	if (!res)
> -		return NULL;
>   
>   	res->name = name;
>   	res->start = start;
>   	res->end = start + n - 1;
>   
> -	write_lock(&resource_lock);
> -
>   	for (;;) {
>   		struct resource *conflict;
>   
> @@ -1230,14 +1224,37 @@ struct resource * __request_region(struct resource *parent,
>   			write_lock(&resource_lock);
>   			continue;
>   		}
> +		return false;
> +	}
> +
> +	return true;
> +}
> +
> +/**
> + * __request_region - create a new busy resource region
> + * @parent: parent resource descriptor
> + * @start: resource start address
> + * @n: resource region size
> + * @name: reserving caller's ID string
> + * @flags: IO resource flags
> + */
> +struct resource *__request_region(struct resource *parent,
> +				  resource_size_t start, resource_size_t n,
> +				  const char *name, int flags)
> +{
> +	struct resource *res = alloc_resource(GFP_KERNEL);
> +
> +	if (!res)
> +		return NULL;
> +
> +	write_lock(&resource_lock);
> +	if (!request_region_locked(parent, res, start, n, name, flags)) {
>   		/* Uhhuh, that didn't work out.. */
>   		free_resource(res);
>   		res = NULL;
> -		break;
>   	}
>   	write_unlock(&resource_lock);
> -
> -	if (res && orig_parent == &iomem_resource)
> +	if (res && parent == &iomem_resource)
>   		revoke_iomem(res);
>   
>   	return res;
> @@ -1779,26 +1796,51 @@ static struct resource *__request_free_mem_region(struct device *dev,
>   {
>   	resource_size_t end, addr;
>   	struct resource *res;
> +	struct region_devres *dr = NULL;
> +
> +	res = alloc_resource(GFP_KERNEL);
> +	if (!res)
> +		return ERR_PTR(-ENOMEM);
> +
> +	if (dev) {
> +		dr = devres_alloc(devm_region_release, sizeof(struct region_devres),
> +				  GFP_KERNEL);
> +		if (!dr) {
> +			free_resource(res);
> +			return ERR_PTR(-ENOMEM);
> +		}
> +	}
>   
>   	size = ALIGN(size, 1UL << PA_SECTION_SHIFT);
>   	end = min_t(unsigned long, base->end, (1UL << MAX_PHYSMEM_BITS) - 1);
>   	addr = end - size + 1UL;
>   
> +	write_lock(&resource_lock);
>   	for (; addr > size && addr >= base->start; addr -= size) {
> -		if (region_intersects(addr, size, 0, IORES_DESC_NONE) !=
> +		if (__region_intersects(addr, size, 0, IORES_DESC_NONE) !=
>   				REGION_DISJOINT)
>   			continue;
>   
> -		if (dev)
> -			res = devm_request_mem_region(dev, addr, size, name);
> -		else
> -			res = request_mem_region(addr, size, name);
> -		if (!res)
> -			return ERR_PTR(-ENOMEM);
> +		if (!request_region_locked(&iomem_resource, res, addr,
> +						   size, name, 0))
> +			break;
> +
>   		res->desc = IORES_DESC_DEVICE_PRIVATE_MEMORY;
> +		if (dev) {
> +			dr->parent = &iomem_resource;
> +			dr->start = addr;
> +			dr->n = size;
> +			devres_add(dev, dr);
> +		}
> +
> +		write_unlock(&resource_lock);
> +		revoke_iomem(res);

This is new, and not mentioned in the commit log, and therefore quite
surprising. It seems like the right thing to do but it also seems like a
different fix! I'm not saying that it should be a separate patch, but it
does seem worth loudly mentioning in the commit log, yes?

>   		return res;
>   	}
>   
> +	write_unlock(&resource_lock);
> +	free_resource(res);
> +
>   	return ERR_PTR(-ERANGE);
>   }
>   
> 

thanks,
-- 
John Hubbard
NVIDIA