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SubjectRe: [PATCH kernel v11 09/34] vfio: powerpc/spapr: Move locked_vm accounting to helpers
On 06/01/2015 02:28 PM, David Gibson wrote:
> On Fri, May 29, 2015 at 06:44:33PM +1000, Alexey Kardashevskiy wrote:
>> There moves locked pages accounting to helpers.
>> Later they will be reused for Dynamic DMA windows (DDW).
>>
>> This reworks debug messages to show the current value and the limit.
>>
>> This stores the locked pages number in the container so when unlocking
>> the iommu table pointer won't be needed. This does not have an effect
>> now but it will with the multiple tables per container as then we will
>> allow attaching/detaching groups on fly and we may end up having
>> a container with no group attached but with the counter incremented.
>>
>> While we are here, update the comment explaining why RLIMIT_MEMLOCK
>> might be required to be bigger than the guest RAM. This also prints
>> pid of the current process in pr_warn/pr_debug.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>> [aw: for the vfio related changes]
>> Acked-by: Alex Williamson <alex.williamson@redhat.com>
>> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
>> Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
>> ---
>> Changes:
>> v4:
>> * new helpers do nothing if @npages == 0
>> * tce_iommu_disable() now can decrement the counter if the group was
>> detached (not possible now but will be in the future)
>> ---
>> drivers/vfio/vfio_iommu_spapr_tce.c | 82 ++++++++++++++++++++++++++++---------
>> 1 file changed, 63 insertions(+), 19 deletions(-)
>>
>> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
>> index 64300cc..40583f9 100644
>> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
>> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
>> @@ -29,6 +29,51 @@
>> static void tce_iommu_detach_group(void *iommu_data,
>> struct iommu_group *iommu_group);
>>
>> +static long try_increment_locked_vm(long npages)
>> +{
>> + long ret = 0, locked, lock_limit;
>> +
>> + if (!current || !current->mm)
>> + return -ESRCH; /* process exited */
>> +
>> + if (!npages)
>> + return 0;
>> +
>> + down_write(&current->mm->mmap_sem);
>> + locked = current->mm->locked_vm + npages;
>
> Is there a possibility of userspace triggering an integer overflow
> here, if npages is really huge?


I do not see how. I just do not accept npages bigger than the host RAM size
in pages. And it is "long". For (lets say) 128GB host, the number of 4KB
pages is (128<<30)/4096=33554432.


>
>> + lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
>> + if (locked > lock_limit && !capable(CAP_IPC_LOCK))
>> + ret = -ENOMEM;
>> + else
>> + current->mm->locked_vm += npages;
>> +
>> + pr_debug("[%d] RLIMIT_MEMLOCK +%ld %ld/%ld%s\n", current->pid,
>> + npages << PAGE_SHIFT,
>> + current->mm->locked_vm << PAGE_SHIFT,
>> + rlimit(RLIMIT_MEMLOCK),
>> + ret ? " - exceeded" : "");
>> +
>> + up_write(&current->mm->mmap_sem);
>> +
>> + return ret;
>> +}
>> +
>> +static void decrement_locked_vm(long npages)
>> +{
>> + if (!current || !current->mm || !npages)
>> + return; /* process exited */
>> +
>> + down_write(&current->mm->mmap_sem);
>> + if (npages > current->mm->locked_vm)
>> + npages = current->mm->locked_vm;
>
> Can this case ever occur (without there being a leak bug somewhere
> else in the code)?


It should not. Safety measure. Having a warning here might make sense but I
believe if this happens, there will be many, many warnings in other places :)



>> + current->mm->locked_vm -= npages;
>> + pr_debug("[%d] RLIMIT_MEMLOCK -%ld %ld/%ld\n", current->pid,
>> + npages << PAGE_SHIFT,
>> + current->mm->locked_vm << PAGE_SHIFT,
>> + rlimit(RLIMIT_MEMLOCK));
>> + up_write(&current->mm->mmap_sem);
>> +}
>> +
>> /*
>> * VFIO IOMMU fd for SPAPR_TCE IOMMU implementation
>> *
>> @@ -45,6 +90,7 @@ struct tce_container {
>> struct mutex lock;
>> struct iommu_table *tbl;
>> bool enabled;
>> + unsigned long locked_pages;
>> };
>>
>> static bool tce_page_is_contained(struct page *page, unsigned page_shift)
>> @@ -60,7 +106,7 @@ static bool tce_page_is_contained(struct page *page, unsigned page_shift)
>> static int tce_iommu_enable(struct tce_container *container)
>> {
>> int ret = 0;
>> - unsigned long locked, lock_limit, npages;
>> + unsigned long locked;
>> struct iommu_table *tbl = container->tbl;
>>
>> if (!container->tbl)
>> @@ -89,21 +135,22 @@ static int tce_iommu_enable(struct tce_container *container)
>> * Also we don't have a nice way to fail on H_PUT_TCE due to ulimits,
>> * that would effectively kill the guest at random points, much better
>> * enforcing the limit based on the max that the guest can map.
>> + *
>> + * Unfortunately at the moment it counts whole tables, no matter how
>> + * much memory the guest has. I.e. for 4GB guest and 4 IOMMU groups
>> + * each with 2GB DMA window, 8GB will be counted here. The reason for
>> + * this is that we cannot tell here the amount of RAM used by the guest
>> + * as this information is only available from KVM and VFIO is
>> + * KVM agnostic.
>> */
>> - down_write(&current->mm->mmap_sem);
>> - npages = (tbl->it_size << tbl->it_page_shift) >> PAGE_SHIFT;
>> - locked = current->mm->locked_vm + npages;
>> - lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
>> - if (locked > lock_limit && !capable(CAP_IPC_LOCK)) {
>> - pr_warn("RLIMIT_MEMLOCK (%ld) exceeded\n",
>> - rlimit(RLIMIT_MEMLOCK));
>> - ret = -ENOMEM;
>> - } else {
>> + locked = (tbl->it_size << tbl->it_page_shift) >> PAGE_SHIFT;
>> + ret = try_increment_locked_vm(locked);
>> + if (ret)
>> + return ret;
>>
>> - current->mm->locked_vm += npages;
>> - container->enabled = true;
>> - }
>> - up_write(&current->mm->mmap_sem);
>> + container->locked_pages = locked;
>> +
>> + container->enabled = true;
>>
>> return ret;
>> }
>> @@ -115,13 +162,10 @@ static void tce_iommu_disable(struct tce_container *container)
>>
>> container->enabled = false;
>>
>> - if (!container->tbl || !current->mm)
>> + if (!current->mm)
>> return;
>>
>> - down_write(&current->mm->mmap_sem);
>> - current->mm->locked_vm -= (container->tbl->it_size <<
>> - container->tbl->it_page_shift) >> PAGE_SHIFT;
>> - up_write(&current->mm->mmap_sem);
>> + decrement_locked_vm(container->locked_pages);
>> }
>>
>> static void *tce_iommu_open(unsigned long arg)
>


--
Alexey


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