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From: Alex Williamson <alex.williamson@redhat.com>
To: John Hubbard <jhubbard@nvidia.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Björn Töpel" <bjorn.topel@intel.com>,
	"Christoph Hellwig" <hch@infradead.org>,
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	"Daniel Vetter" <daniel@ffwll.ch>,
	"Dave Chinner" <david@fromorbit.com>,
	"David Airlie" <airlied@linux.ie>,
	"David S . Miller" <davem@davemloft.net>,
	"Ira Weiny" <ira.weiny@intel.com>, "Jan Kara" <jack@suse.cz>,
	"Jason Gunthorpe" <jgg@ziepe.ca>, "Jens Axboe" <axboe@kernel.dk>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Magnus Karlsson" <magnus.karlsson@intel.com>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
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	netdev@vger.kernel.org, linux-mm@kvack.org,
	LKML <linux-kernel@vger.kernel.org>,
	"Jason Gunthorpe" <jgg@mellanox.com>
Subject: Re: [PATCH v7 09/24] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM
Date: Thu, 21 Nov 2019 14:35:25 -0700
Message-ID: <20191121143525.50deb72f@x1.home> (raw)
In-Reply-To: <20191121071354.456618-10-jhubbard@nvidia.com>

On Wed, 20 Nov 2019 23:13:39 -0800
John Hubbard <jhubbard@nvidia.com> wrote:

> As it says in the updated comment in gup.c: current FOLL_LONGTERM
> behavior is incompatible with FAULT_FLAG_ALLOW_RETRY because of the
> FS DAX check requirement on vmas.
> 
> However, the corresponding restriction in get_user_pages_remote() was
> slightly stricter than is actually required: it forbade all
> FOLL_LONGTERM callers, but we can actually allow FOLL_LONGTERM callers
> that do not set the "locked" arg.
> 
> Update the code and comments accordingly, and update the VFIO caller
> to take advantage of this, fixing a bug as a result: the VFIO caller
> is logically a FOLL_LONGTERM user.
> 
> Also, remove an unnessary pair of calls that were releasing and
> reacquiring the mmap_sem. There is no need to avoid holding mmap_sem
> just in order to call page_to_pfn().
> 
> Also, move the DAX check ("if a VMA is DAX, don't allow long term
> pinning") from the VFIO call site, all the way into the internals
> of get_user_pages_remote() and __gup_longterm_locked(). That is:
> get_user_pages_remote() calls __gup_longterm_locked(), which in turn
> calls check_dax_vmas(). It's lightly explained in the comments as well.
> 
> Thanks to Jason Gunthorpe for pointing out a clean way to fix this,
> and to Dan Williams for helping clarify the DAX refactoring.
> 
> Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Jerome Glisse <jglisse@redhat.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
>  drivers/vfio/vfio_iommu_type1.c | 30 +++++-------------------------
>  mm/gup.c                        | 27 ++++++++++++++++++++++-----
>  2 files changed, 27 insertions(+), 30 deletions(-)

Tested with device assignment and Intel mdev vGPU assignment with QEMU
userspace:

Tested-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>

Feel free to include for 19/24 as well.  Thanks,

Alex

> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index d864277ea16f..c7a111ad9975 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -340,7 +340,6 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
>  {
>  	struct page *page[1];
>  	struct vm_area_struct *vma;
> -	struct vm_area_struct *vmas[1];
>  	unsigned int flags = 0;
>  	int ret;
>  
> @@ -348,33 +347,14 @@ 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]);
> -		}
> -	}
> -	up_read(&mm->mmap_sem);
> -
> +	ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
> +				    page, NULL, NULL);
>  	if (ret == 1) {
>  		*pfn = page_to_pfn(page[0]);
> -		return 0;
> +		ret = 0;
> +		goto done;
>  	}
>  
> -	down_read(&mm->mmap_sem);
> -
>  	vaddr = untagged_addr(vaddr);
>  
>  	vma = find_vma_intersection(mm, vaddr, vaddr + 1);
> @@ -384,7 +364,7 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
>  		if (is_invalid_reserved_pfn(*pfn))
>  			ret = 0;
>  	}
> -
> +done:
>  	up_read(&mm->mmap_sem);
>  	return ret;
>  }
> diff --git a/mm/gup.c b/mm/gup.c
> index 14fcdc502166..cce2c9676853 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -29,6 +29,13 @@ struct follow_page_context {
>  	unsigned int page_mask;
>  };
>  
> +static __always_inline long __gup_longterm_locked(struct task_struct *tsk,
> +						  struct mm_struct *mm,
> +						  unsigned long start,
> +						  unsigned long nr_pages,
> +						  struct page **pages,
> +						  struct vm_area_struct **vmas,
> +						  unsigned int flags);
>  /*
>   * Return the compound head page with ref appropriately incremented,
>   * or NULL if that failed.
> @@ -1167,13 +1174,23 @@ long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
>  		struct vm_area_struct **vmas, int *locked)
>  {
>  	/*
> -	 * FIXME: Current FOLL_LONGTERM behavior is incompatible with
> +	 * Parts of FOLL_LONGTERM behavior are incompatible with
>  	 * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
> -	 * vmas.  As there are no users of this flag in this call we simply
> -	 * disallow this option for now.
> +	 * vmas. However, this only comes up if locked is set, and there are
> +	 * callers that do request FOLL_LONGTERM, but do not set locked. So,
> +	 * allow what we can.
>  	 */
> -	if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
> -		return -EINVAL;
> +	if (gup_flags & FOLL_LONGTERM) {
> +		if (WARN_ON_ONCE(locked))
> +			return -EINVAL;
> +		/*
> +		 * This will check the vmas (even if our vmas arg is NULL)
> +		 * and return -ENOTSUPP if DAX isn't allowed in this case:
> +		 */
> +		return __gup_longterm_locked(tsk, mm, start, nr_pages, pages,
> +					     vmas, gup_flags | FOLL_TOUCH |
> +					     FOLL_REMOTE);
> +	}
>  
>  	return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,
>  				       locked,



  parent reply index

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-21  7:13 [PATCH v7 00/24] mm/gup: track dma-pinned pages: FOLL_PIN John Hubbard
2019-11-21  7:13 ` [PATCH v7 01/24] mm/gup: pass flags arg to __gup_device_* functions John Hubbard
2019-11-21  8:06   ` Christoph Hellwig
2019-11-21  8:25     ` John Hubbard
2019-11-21  7:13 ` [PATCH v7 02/24] mm/gup: factor out duplicate code from four routines John Hubbard
2019-11-21  8:03   ` Christoph Hellwig
2019-11-21  8:29     ` John Hubbard
2019-11-21  9:49       ` Jan Kara
2019-11-21 21:47         ` John Hubbard
2019-11-21  9:54       ` Jan Kara
2019-11-22  2:54         ` John Hubbard
2019-11-22 11:15           ` Jan Kara
2019-11-21  7:13 ` [PATCH v7 03/24] mm/gup: move try_get_compound_head() to top, fix minor issues John Hubbard
2019-11-21  8:07   ` Christoph Hellwig
2019-11-21  7:13 ` [PATCH v7 04/24] mm: Cleanup __put_devmap_managed_page() vs ->page_free() John Hubbard
2019-11-21  8:04   ` Christoph Hellwig
2019-11-21  7:13 ` [PATCH v7 05/24] mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages John Hubbard
2019-11-21  8:05   ` Christoph Hellwig
2019-11-21  8:54     ` John Hubbard
2019-11-21 16:59       ` Dan Williams
2019-11-21 22:22         ` John Hubbard
2019-11-21  7:13 ` [PATCH v7 06/24] goldish_pipe: rename local pin_user_pages() routine John Hubbard
2019-11-21  8:08   ` Christoph Hellwig
2019-11-21  8:36     ` John Hubbard
2019-11-21  7:13 ` [PATCH v7 07/24] IB/umem: use get_user_pages_fast() to pin DMA pages John Hubbard
2019-11-21  8:07   ` Christoph Hellwig
2019-11-21 14:36     ` Jason Gunthorpe
2019-11-24  6:14       ` John Hubbard
2019-11-24 10:07       ` Leon Romanovsky
2019-11-25  0:05         ` John Hubbard
2019-11-25  0:53           ` Jason Gunthorpe
2019-11-21  7:13 ` [PATCH v7 08/24] media/v4l2-core: set pages dirty upon releasing DMA buffers John Hubbard
2019-11-21  8:09   ` Christoph Hellwig
2019-11-21  7:13 ` [PATCH v7 09/24] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM John Hubbard
2019-11-21  8:10   ` Christoph Hellwig
2019-11-21  8:48     ` John Hubbard
2019-11-21 21:35   ` Alex Williamson [this message]
2019-11-21 21:49     ` John Hubbard
2019-11-21  7:13 ` [PATCH v7 10/24] mm/gup: introduce pin_user_pages*() and FOLL_PIN John Hubbard
2019-11-21  7:13 ` [PATCH v7 11/24] goldish_pipe: convert to pin_user_pages() and put_user_page() John Hubbard
2019-11-21  7:13 ` [PATCH v7 12/24] IB/{core,hw,umem}: set FOLL_PIN via pin_user_pages*(), fix up ODP John Hubbard
2019-11-21  7:13 ` [PATCH v7 13/24] mm/process_vm_access: set FOLL_PIN via pin_user_pages_remote() John Hubbard
2019-11-21  7:13 ` [PATCH v7 14/24] drm/via: set FOLL_PIN via pin_user_pages_fast() John Hubbard
2019-11-21  7:13 ` [PATCH v7 15/24] fs/io_uring: set FOLL_PIN via pin_user_pages() John Hubbard
2019-11-21  7:13 ` [PATCH v7 16/24] net/xdp: " John Hubbard
2019-11-21  7:13 ` [PATCH v7 17/24] mm/gup: track FOLL_PIN pages John Hubbard
2019-11-21  9:39   ` Jan Kara
2019-11-30 18:59   ` kbuild test robot
2019-11-21  7:13 ` [PATCH v7 18/24] media/v4l2-core: pin_user_pages (FOLL_PIN) and put_user_page() conversion John Hubbard
2019-11-21  7:13 ` [PATCH v7 19/24] vfio, mm: " John Hubbard
2019-11-21  7:13 ` [PATCH v7 20/24] powerpc: book3s64: convert to pin_user_pages() and put_user_page() John Hubbard
2019-11-21  7:13 ` [PATCH v7 21/24] mm/gup_benchmark: use proper FOLL_WRITE flags instead of hard-coding "1" John Hubbard
2019-11-21  7:13 ` [PATCH v7 22/24] mm/gup_benchmark: support pin_user_pages() and related calls John Hubbard
2019-11-21  7:13 ` [PATCH v7 23/24] selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN coverage John Hubbard
2019-11-21  7:13 ` [PATCH v7 24/24] mm, tree-wide: rename put_user_page*() to unpin_user_page*() John Hubbard

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