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Message-ID: <5e478ac6-814d-d599-e44f-5e90232d30b3@ozlabs.ru>
Date: Wed, 13 Feb 2019 11:34:28 +1100
From: Alexey Kardashevskiy <aik@...abs.ru>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: Daniel Jordan <daniel.m.jordan@...cle.com>, jgg@...pe.ca,
akpm@...ux-foundation.org, dave@...olabs.net, jack@...e.cz,
cl@...ux.com, linux-mm@...ck.org, kvm@...r.kernel.org,
kvm-ppc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
linux-fpga@...r.kernel.org, linux-kernel@...r.kernel.org,
paulus@...abs.org, benh@...nel.crashing.org, mpe@...erman.id.au,
hao.wu@...el.com, atull@...nel.org, mdf@...nel.org
Subject: Re: [PATCH 2/5] vfio/spapr_tce: use pinned_vm instead of locked_vm to
account pinned pages
On 13/02/2019 05:56, Alex Williamson wrote:
> On Tue, 12 Feb 2019 17:56:18 +1100
> Alexey Kardashevskiy <aik@...abs.ru> wrote:
>
>> 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@...cle.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.
>
> I don't know of a ulimit only covering pinned pages, so for
> documentation it seems more correct to continue referring to this as
> locked page accounting.
>
>>> @@ -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,
>
> The first 2 look pretty sketchy to me, is there a case where you don't
> know how many pages you've pinned to unpin them?
No case like this, this is why WARN_ON_ONCE(). At the time I could have
been under impression that pinned_vm is system-global, hence that
adjustment but we do not really need it there.
> And can it ever
> really be correct to just unpin whatever remains? The last access is
> diagnostic, which leaves 1. Daniel's rework to warn on a negative
> result looks more sane. Thanks,
Yes it does look sane.
--
Alexey
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