linux-kernel - Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL

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Message-ID: <471e513c-833f-2f8b-60db-5d9c56a8f766@nvidia.com>
Date:   Tue, 12 Nov 2019 14:24:32 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
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        Dave Chinner <david@...morbit.com>,
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Subject: Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and
 FOLL_LONGTERM

On 11/12/19 1:57 PM, Dan Williams wrote:
...
>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>> index d864277ea16f..017689b7c32b 100644
>> --- a/drivers/vfio/vfio_iommu_type1.c
>> +++ b/drivers/vfio/vfio_iommu_type1.c
>> @@ -348,24 +348,20 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
>>                 flags |= FOLL_WRITE;
>>
>>         down_read(&mm->mmap_sem);
>> -       if (mm == current->mm) {
>> -               ret = get_user_pages(vaddr, 1, flags | FOLL_LONGTERM, page,
>> -                                    vmas);
>> -       } else {
>> -               ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page,
>> -                                           vmas, NULL);
>> -               /*
>> -                * The lifetime of a vaddr_get_pfn() page pin is
>> -                * userspace-controlled. In the fs-dax case this could
>> -                * lead to indefinite stalls in filesystem operations.
>> -                * Disallow attempts to pin fs-dax pages via this
>> -                * interface.
>> -                */
>> -               if (ret > 0 && vma_is_fsdax(vmas[0])) {
>> -                       ret = -EOPNOTSUPP;
>> -                       put_page(page[0]);
>> -               }
>> +       ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
>> +                                   page, vmas, NULL);
> 
> Hmm, what's the point of passing FOLL_LONGTERM to
> get_user_pages_remote() if get_user_pages_remote() is not going to
> check the vma? I think we got to this code state because the

FOLL_LONGTERM is short-lived in this location, because patch 23 
("mm/gup: remove support for gup(FOLL_LONGTERM)") removes it, after
callers are changed over to pin_longterm_pages*().

So FOLL_LONGTERM is not doing much now, but it is basically a marker for
"change gup(FOLL_LONGTERM) to pin_longterm_pages()", and patch 18
actually makes that change.

And then pin_longterm_pages*() is, in turn, a way to mark all the 
places that need file system and/or user space interactions (layout
leases, etc), as per "Case 2: RDMA" in the new 
Documentation/vm/pin_user_pages.rst.

> get_user_pages() vs get_user_pages_remote() split predated the
> introduction of FOLL_LONGTERM.

Yes. And I do want clean this up as I go, so we don't end up with
stale concepts lingering in gup.c...

> 
> I think check_vma_flags() should do the ((FOLL_LONGTERM | FOLL_GET) &&
> vma_is_fsdax()) check and that would also remove the need for
> __gup_longterm_locked.
> 

Good idea, but there is still the call to check_and_migrate_cma_pages(), 
inside __gup_longterm_locked().  So it's a little more involved and
we can't trivially delete __gup_longterm_locked() yet, right?


thanks,
-- 
John Hubbard
NVIDIA