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SubjectRe: [PATCH 2/5] vfio/spapr_tce: use pinned_vm instead of locked_vm to account pinned pages
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On 12/02/2019 09:44, Daniel Jordan wrote:
> Beginning with bc3e53f682d9 ("mm: distinguish between mlocked and pinned
> pages"), locked and pinned pages are accounted separately. The SPAPR
> TCE VFIO IOMMU driver accounts pinned pages to locked_vm; use pinned_vm
> instead.
>
> pinned_vm recently became atomic and so no longer relies on mmap_sem
> held as writer: delete.
>
> Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
> ---
> Documentation/vfio.txt | 6 +--
> drivers/vfio/vfio_iommu_spapr_tce.c | 64 ++++++++++++++---------------
> 2 files changed, 33 insertions(+), 37 deletions(-)
>
> diff --git a/Documentation/vfio.txt b/Documentation/vfio.txt
> index f1a4d3c3ba0b..fa37d65363f9 100644
> --- a/Documentation/vfio.txt
> +++ b/Documentation/vfio.txt
> @@ -308,7 +308,7 @@ This implementation has some specifics:
> currently there is no way to reduce the number of calls. In order to make
> things faster, the map/unmap handling has been implemented in real mode
> which provides an excellent performance which has limitations such as
> - inability to do locked pages accounting in real time.
> + inability to do pinned pages accounting in real time.
>
> 4) According to sPAPR specification, A Partitionable Endpoint (PE) is an I/O
> subtree that can be treated as a unit for the purposes of partitioning and
> @@ -324,7 +324,7 @@ This implementation has some specifics:
> returns the size and the start of the DMA window on the PCI bus.
>
> VFIO_IOMMU_ENABLE
> - enables the container. The locked pages accounting
> + enables the container. The pinned pages accounting
> is done at this point. This lets user first to know what
> the DMA window is and adjust rlimit before doing any real job.
>
> @@ -454,7 +454,7 @@ This implementation has some specifics:
>
> PPC64 paravirtualized guests generate a lot of map/unmap requests,
> and the handling of those includes pinning/unpinning pages and updating
> - mm::locked_vm counter to make sure we do not exceed the rlimit.
> + mm::pinned_vm counter to make sure we do not exceed the rlimit.
> The v2 IOMMU splits accounting and pinning into separate operations:
>
> - VFIO_IOMMU_SPAPR_REGISTER_MEMORY/VFIO_IOMMU_SPAPR_UNREGISTER_MEMORY ioctls
> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
> index c424913324e3..f47e020dc5e4 100644
> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
> @@ -34,9 +34,11 @@
> static void tce_iommu_detach_group(void *iommu_data,
> struct iommu_group *iommu_group);
>
> -static long try_increment_locked_vm(struct mm_struct *mm, long npages)
> +static long try_increment_pinned_vm(struct mm_struct *mm, long npages)
> {
> - long ret = 0, locked, lock_limit;
> + long ret = 0;
> + s64 pinned;
> + unsigned long lock_limit;
>
> if (WARN_ON_ONCE(!mm))
> return -EPERM;
> @@ -44,39 +46,33 @@ static long try_increment_locked_vm(struct mm_struct *mm, long npages)
> if (!npages)
> return 0;
>
> - down_write(&mm->mmap_sem);
> - locked = mm->locked_vm + npages;
> + pinned = atomic64_add_return(npages, &mm->pinned_vm);
> lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> - if (locked > lock_limit && !capable(CAP_IPC_LOCK))
> + if (pinned > lock_limit && !capable(CAP_IPC_LOCK)) {
> ret = -ENOMEM;
> - else
> - mm->locked_vm += npages;
> + atomic64_sub(npages, &mm->pinned_vm);
> + }
>
> - pr_debug("[%d] RLIMIT_MEMLOCK +%ld %ld/%ld%s\n", current->pid,
> + pr_debug("[%d] RLIMIT_MEMLOCK +%ld %ld/%lu%s\n", current->pid,
> npages << PAGE_SHIFT,
> - mm->locked_vm << PAGE_SHIFT,
> - rlimit(RLIMIT_MEMLOCK),
> - ret ? " - exceeded" : "");
> -
> - up_write(&mm->mmap_sem);
> + atomic64_read(&mm->pinned_vm) << PAGE_SHIFT,
> + rlimit(RLIMIT_MEMLOCK), ret ? " - exceeded" : "");
>
> return ret;
> }
>
> -static void decrement_locked_vm(struct mm_struct *mm, long npages)
> +static void decrement_pinned_vm(struct mm_struct *mm, long npages)
> {
> if (!mm || !npages)
> return;
>
> - down_write(&mm->mmap_sem);
> - if (WARN_ON_ONCE(npages > mm->locked_vm))
> - npages = mm->locked_vm;
> - mm->locked_vm -= npages;
> - pr_debug("[%d] RLIMIT_MEMLOCK -%ld %ld/%ld\n", current->pid,
> + if (WARN_ON_ONCE(npages > atomic64_read(&mm->pinned_vm)))
> + npages = atomic64_read(&mm->pinned_vm);
> + atomic64_sub(npages, &mm->pinned_vm);
> + pr_debug("[%d] RLIMIT_MEMLOCK -%ld %ld/%lu\n", current->pid,
> npages << PAGE_SHIFT,
> - mm->locked_vm << PAGE_SHIFT,
> + atomic64_read(&mm->pinned_vm) << PAGE_SHIFT,
> rlimit(RLIMIT_MEMLOCK));
> - up_write(&mm->mmap_sem);


So it used to be down_write+up_write and stuff in between.

Now it is 3 independent accesses (actually 4 but the last one is
diagnostic) with no locking around them. Why do not we need a lock
anymore precisely? Thanks,




> }
>
> /*
> @@ -110,7 +106,7 @@ struct tce_container {
> bool enabled;
> bool v2;
> bool def_window_pending;
> - unsigned long locked_pages;
> + unsigned long pinned_pages;
> struct mm_struct *mm;
> struct iommu_table *tables[IOMMU_TABLE_GROUP_MAX_TABLES];
> struct list_head group_list;
> @@ -283,7 +279,7 @@ static int tce_iommu_find_free_table(struct tce_container *container)
> static int tce_iommu_enable(struct tce_container *container)
> {
> int ret = 0;
> - unsigned long locked;
> + unsigned long pinned;
> struct iommu_table_group *table_group;
> struct tce_iommu_group *tcegrp;
>
> @@ -292,15 +288,15 @@ static int tce_iommu_enable(struct tce_container *container)
>
> /*
> * When userspace pages are mapped into the IOMMU, they are effectively
> - * locked memory, so, theoretically, we need to update the accounting
> - * of locked pages on each map and unmap. For powerpc, the map unmap
> + * pinned memory, so, theoretically, we need to update the accounting
> + * of pinned pages on each map and unmap. For powerpc, the map unmap
> * paths can be very hot, though, and the accounting would kill
> * performance, especially since it would be difficult to impossible
> * to handle the accounting in real mode only.
> *
> * To address that, rather than precisely accounting every page, we
> - * instead account for a worst case on locked memory when the iommu is
> - * enabled and disabled. The worst case upper bound on locked memory
> + * instead account for a worst case on pinned memory when the iommu is
> + * enabled and disabled. The worst case upper bound on pinned memory
> * is the size of the whole iommu window, which is usually relatively
> * small (compared to total memory sizes) on POWER hardware.
> *
> @@ -317,7 +313,7 @@ static int tce_iommu_enable(struct tce_container *container)
> *
> * So we do not allow enabling a container without a group attached
> * as there is no way to know how much we should increment
> - * the locked_vm counter.
> + * the pinned_vm counter.
> */
> if (!tce_groups_attached(container))
> return -ENODEV;
> @@ -335,12 +331,12 @@ static int tce_iommu_enable(struct tce_container *container)
> if (ret)
> return ret;
>
> - locked = table_group->tce32_size >> PAGE_SHIFT;
> - ret = try_increment_locked_vm(container->mm, locked);
> + pinned = table_group->tce32_size >> PAGE_SHIFT;
> + ret = try_increment_pinned_vm(container->mm, pinned);
> if (ret)
> return ret;
>
> - container->locked_pages = locked;
> + container->pinned_pages = pinned;
>
> container->enabled = true;
>
> @@ -355,7 +351,7 @@ static void tce_iommu_disable(struct tce_container *container)
> container->enabled = false;
>
> BUG_ON(!container->mm);
> - decrement_locked_vm(container->mm, container->locked_pages);
> + decrement_pinned_vm(container->mm, container->pinned_pages);
> }
>
> static void *tce_iommu_open(unsigned long arg)
> @@ -658,7 +654,7 @@ static long tce_iommu_create_table(struct tce_container *container,
> if (!table_size)
> return -EINVAL;
>
> - ret = try_increment_locked_vm(container->mm, table_size >> PAGE_SHIFT);
> + ret = try_increment_pinned_vm(container->mm, table_size >> PAGE_SHIFT);
> if (ret)
> return ret;
>
> @@ -677,7 +673,7 @@ static void tce_iommu_free_table(struct tce_container *container,
> unsigned long pages = tbl->it_allocated_size >> PAGE_SHIFT;
>
> iommu_tce_table_put(tbl);
> - decrement_locked_vm(container->mm, pages);
> + decrement_pinned_vm(container->mm, pages);
> }
>
> static long tce_iommu_create_window(struct tce_container *container,
>

--
Alexey

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