[PATCH] mm/cma: Make kmemleak ignore CMA regions

[PATCH] mm/cma: Make kmemleak ignore CMA regions

[Thierry Reding]

From: Thierry Reding <treding@nvidia.com>

kmemleak will add allocations as objects to a pool. The memory allocated
for each object in this pool is periodically searched for pointers to
other allocated objects. This only works for memory that is mapped into
the kernel's virtual address space, which happens not to be the case for
most CMA regions.

Furthermore, CMA regions are typically used to store data transferred to
or from a device and therefore don't contain pointers to other objects.

Signed-off-by: Thierry Reding <treding@nvidia.com>
---
Note: I'm not sure this is really the right fix. But without this, the
kernel crashes on the first execution of the scan_gray_list() because
it tries to access highmem. Perhaps a more appropriate fix would be to
reject any object that can't map to a kernel virtual address?
---
 mm/cma.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/cma.c b/mm/cma.c
index 963bc4add9af..349f9266f6d3 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -280,6 +280,7 @@ int __init cma_declare_contiguous(phys_addr_t base,
 			ret = -ENOMEM;
 			goto err;
 		} else {
+			kmemleak_ignore(phys_to_virt(addr));
 			base = addr;
 		}
 	}
-- 
2.1.2

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Re: [PATCH] mm/cma: Make kmemleak ignore CMA regions

[Andrew Morton]

On Tue, 21 Oct 2014 14:14:56 +0200 Thierry Reding <thierry.reding@gmail.com> wrote:

> From: Thierry Reding <treding@nvidia.com>
> 
> kmemleak will add allocations as objects to a pool. The memory allocated
> for each object in this pool is periodically searched for pointers to
> other allocated objects. This only works for memory that is mapped into
> the kernel's virtual address space, which happens not to be the case for
> most CMA regions.
> 
> Furthermore, CMA regions are typically used to store data transferred to
> or from a device and therefore don't contain pointers to other objects.
> 
> Signed-off-by: Thierry Reding <treding@nvidia.com>
> ---
> Note: I'm not sure this is really the right fix. But without this, the
> kernel crashes on the first execution of the scan_gray_list() because
> it tries to access highmem. Perhaps a more appropriate fix would be to
> reject any object that can't map to a kernel virtual address?

Let's cc Catalin.

> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -280,6 +280,7 @@ int __init cma_declare_contiguous(phys_addr_t base,
>  			ret = -ENOMEM;
>  			goto err;
>  		} else {
> +			kmemleak_ignore(phys_to_virt(addr));
>  			base = addr;
>  		}
>  	}

And let's tell our poor readers why we did stuff.  Something like this.

--- a/mm/cma.c~mm-cma-make-kmemleak-ignore-cma-regions-fix
+++ a/mm/cma.c
@@ -280,6 +280,10 @@ int __init cma_declare_contiguous(phys_a
 			ret = -ENOMEM;
 			goto err;
 		} else {
+			/*
+			 * kmemleak writes metadata to the tracked objects, but
+			 * this address isn't mapped and accessible.
+			 */
 			kmemleak_ignore(phys_to_virt(addr));
 			base = addr;
 		}
_

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Re: [PATCH] mm/cma: Make kmemleak ignore CMA regions

[Catalin Marinas]

On 23 October 2014 21:22, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Tue, 21 Oct 2014 14:14:56 +0200 Thierry Reding <thierry.reding@gmail.com> wrote:
>
>> From: Thierry Reding <treding@nvidia.com>
>>
>> kmemleak will add allocations as objects to a pool. The memory allocated
>> for each object in this pool is periodically searched for pointers to
>> other allocated objects. This only works for memory that is mapped into
>> the kernel's virtual address space, which happens not to be the case for
>> most CMA regions.
>>
>> Furthermore, CMA regions are typically used to store data transferred to
>> or from a device and therefore don't contain pointers to other objects.
>>
>> Signed-off-by: Thierry Reding <treding@nvidia.com>
>> ---
>> Note: I'm not sure this is really the right fix. But without this, the
>> kernel crashes on the first execution of the scan_gray_list() because
>> it tries to access highmem. Perhaps a more appropriate fix would be to
>> reject any object that can't map to a kernel virtual address?
>
> Let's cc Catalin.
>
>> --- a/mm/cma.c
>> +++ b/mm/cma.c
>> @@ -280,6 +280,7 @@ int __init cma_declare_contiguous(phys_addr_t base,
>>                       ret = -ENOMEM;
>>                       goto err;
>>               } else {
>> +                     kmemleak_ignore(phys_to_virt(addr));
>>                       base = addr;
>>               }
>>       }

I wonder whether using __va() for the argument of kmemleak_alloc() in
memblock_alloc_range_nid() is always correct. Is
memblock.current_limit guaranteed to be in lowmem? If not, I think we
need some logic not to call kmemleak_alloc() for all memblock
allocations (and avoid the need to ignore them later).

> And let's tell our poor readers why we did stuff.  Something like this.
>
> --- a/mm/cma.c~mm-cma-make-kmemleak-ignore-cma-regions-fix
> +++ a/mm/cma.c
> @@ -280,6 +280,10 @@ int __init cma_declare_contiguous(phys_a
>                         ret = -ENOMEM;
>                         goto err;
>                 } else {
> +                       /*
> +                        * kmemleak writes metadata to the tracked objects, but
> +                        * this address isn't mapped and accessible.
> +                        */
>                         kmemleak_ignore(phys_to_virt(addr));
>                         base = addr;
>                 }

The reason is different, as per Therry's patch description. Kmemleak
does not write metadata to the tracked objects but reads them during
memory scanning. So maybe something like "kmemleak scans/reads tracked
objects for pointers to other objects but this address isn't mapped
and accessible."

A better API to use here would have been kmemleak_no_scan(), however,
I don't think we care about such CMA pointers anyway since they seem
to be tracked by physical address which kmemleak doesn't store.

-- 
Catalin

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