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From: Jerome Glisse <jglisse@redhat.com>
To: John Hubbard <jhubbard@nvidia.com>
Cc: "Michal Hocko" <mhocko@suse.com>, "Jan Kara" <jack@suse.cz>,
	kvm@vger.kernel.org, linux-doc@vger.kernel.org,
	"David Airlie" <airlied@linux.ie>,
	"Dave Chinner" <david@fromorbit.com>,
	dri-devel@lists.freedesktop.org, linux-mm@kvack.org,
	"Paul Mackerras" <paulus@samba.org>,
	linux-kselftest@vger.kernel.org,
	"Ira Weiny" <ira.weiny@intel.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	linux-rdma@vger.kernel.org,
	"Christoph Hellwig" <hch@infradead.org>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	"Björn Töpel" <bjorn.topel@intel.com>,
	linux-media@vger.kernel.org, "Shuah Khan" <shuah@kernel.org>,
	linux-block@vger.kernel.org,
	"Alex Williamson" <alex.williamson@redhat.com>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	bpf@vger.kernel.org,
	"Magnus Karlsson" <magnus.karlsson@intel.com>,
	"Jens Axboe" <axboe@kernel.dk>,
	netdev@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	linux-fsdevel@vger.kernel.org,
	"Andrew Morton" <akpm@linux-foundation.org>,
	linuxppc-dev@lists.ozlabs.org,
	"David S . Miller" <davem@davemloft.net>,
	"Mike Kravetz" <mike.kravetz@oracle.com>
Subject: Re: [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL_PIN
Date: Mon, 4 Nov 2019 12:33:25 -0500
Message-ID: <20191104173325.GD5134@redhat.com> (raw)
In-Reply-To: <20191103211813.213227-6-jhubbard@nvidia.com>

On Sun, Nov 03, 2019 at 01:18:00PM -0800, John Hubbard wrote:
> Introduce pin_user_pages*() variations of get_user_pages*() calls,
> and also pin_longterm_pages*() variations.
> 
> These variants all set FOLL_PIN, which is also introduced, and
> thoroughly documented.
> 
> The pin_longterm*() variants also set FOLL_LONGTERM, in addition
> to FOLL_PIN:
> 
>     pin_user_pages()
>     pin_user_pages_remote()
>     pin_user_pages_fast()
> 
>     pin_longterm_pages()
>     pin_longterm_pages_remote()
>     pin_longterm_pages_fast()
> 
> All pages that are pinned via the above calls, must be unpinned via
> put_user_page().
> 
> The underlying rules are:
> 
> * These are gup-internal flags, so the call sites should not directly
> set FOLL_PIN nor FOLL_LONGTERM. That behavior is enforced with
> assertions, for the new FOLL_PIN flag. However, for the pre-existing
> FOLL_LONGTERM flag, which has some call sites that still directly
> set FOLL_LONGTERM, there is no assertion yet.
> 
> * Call sites that want to indicate that they are going to do DirectIO
>   ("DIO") or something with similar characteristics, should call a
>   get_user_pages()-like wrapper call that sets FOLL_PIN. These wrappers
>   will:
>         * Start with "pin_user_pages" instead of "get_user_pages". That
>           makes it easy to find and audit the call sites.
>         * Set FOLL_PIN
> 
> * For pages that are received via FOLL_PIN, those pages must be returned
>   via put_user_page().
> 
> Thanks to Jan Kara and Vlastimil Babka for explaining the 4 cases
> in this documentation. (I've reworded it and expanded on it slightly.)
> 
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>

Few nitpick belows, nonetheless:

Reviewed-by: Jérôme Glisse <jglisse@redhat.com>

> ---
>  Documentation/vm/index.rst          |   1 +
>  Documentation/vm/pin_user_pages.rst | 212 ++++++++++++++++++++++
>  include/linux/mm.h                  |  62 ++++++-
>  mm/gup.c                            | 265 +++++++++++++++++++++++++---
>  4 files changed, 514 insertions(+), 26 deletions(-)
>  create mode 100644 Documentation/vm/pin_user_pages.rst
> 

[...]

> diff --git a/Documentation/vm/pin_user_pages.rst b/Documentation/vm/pin_user_pages.rst
> new file mode 100644
> index 000000000000..3910f49ca98c
> --- /dev/null
> +++ b/Documentation/vm/pin_user_pages.rst

[...]

> +
> +FOLL_PIN, FOLL_GET, FOLL_LONGTERM: when to use which flags
> +==========================================================
> +
> +Thanks to Jan Kara, Vlastimil Babka and several other -mm people, for describing
> +these categories:
> +
> +CASE 1: Direct IO (DIO)
> +-----------------------
> +There are GUP references to pages that are serving
> +as DIO buffers. These buffers are needed for a relatively short time (so they
> +are not "long term"). No special synchronization with page_mkclean() or
> +munmap() is provided. Therefore, flags to set at the call site are: ::
> +
> +    FOLL_PIN
> +
> +...but rather than setting FOLL_PIN directly, call sites should use one of
> +the pin_user_pages*() routines that set FOLL_PIN.
> +
> +CASE 2: RDMA
> +------------
> +There are GUP references to pages that are serving as DMA
> +buffers. These buffers are needed for a long time ("long term"). No special
> +synchronization with page_mkclean() or munmap() is provided. Therefore, flags
> +to set at the call site are: ::
> +
> +    FOLL_PIN | FOLL_LONGTERM
> +
> +NOTE: Some pages, such as DAX pages, cannot be pinned with longterm pins. That's
> +because DAX pages do not have a separate page cache, and so "pinning" implies
> +locking down file system blocks, which is not (yet) supported in that way.
> +
> +CASE 3: ODP
> +-----------
> +(Mellanox/Infiniband On Demand Paging: the hardware supports
> +replayable page faulting). There are GUP references to pages serving as DMA
> +buffers. For ODP, MMU notifiers are used to synchronize with page_mkclean()
> +and munmap(). Therefore, normal GUP calls are sufficient, so neither flag
> +needs to be set.

I would not include ODP or anything like it here, they do not use
GUP anymore and i believe it is more confusing here. I would how-
ever include some text in this documentation explaining that hard-
ware that support page fault is superior as it does not incur any
of the issues described here.

> +
> +CASE 4: Pinning for struct page manipulation only
> +-------------------------------------------------
> +Here, normal GUP calls are sufficient, so neither flag needs to be set.
> +

[...]

> diff --git a/mm/gup.c b/mm/gup.c
> index 199da99e8ffc..1aea48427879 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c

[...]

> @@ -1014,7 +1018,16 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
>  		BUG_ON(*locked != 1);
>  	}
>  
> -	if (pages)
> +	/*
> +	 * FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior
> +	 * is to set FOLL_GET if the caller wants pages[] filled in (but has
> +	 * carelessly failed to specify FOLL_GET), so keep doing that, but only
> +	 * for FOLL_GET, not for the newer FOLL_PIN.
> +	 *
> +	 * FOLL_PIN always expects pages to be non-null, but no need to assert
> +	 * that here, as any failures will be obvious enough.
> +	 */
> +	if (pages && !(flags & FOLL_PIN))
>  		flags |= FOLL_GET;

Did you look at user that have pages and not FOLL_GET set ?
I believe it would be better to first fix them to end up
with FOLL_GET set and then error out if pages is != NULL but
nor FOLL_GET or FOLL_PIN is set.

>  
>  	pages_done = 0;

> @@ -2373,24 +2402,9 @@ static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
>  	return ret;
>  }
>  
> -/**
> - * get_user_pages_fast() - pin user pages in memory
> - * @start:	starting user address
> - * @nr_pages:	number of pages from start to pin
> - * @gup_flags:	flags modifying pin behaviour
> - * @pages:	array that receives pointers to the pages pinned.
> - *		Should be at least nr_pages long.
> - *
> - * Attempt to pin user pages in memory without taking mm->mmap_sem.
> - * If not successful, it will fall back to taking the lock and
> - * calling get_user_pages().
> - *
> - * Returns number of pages pinned. This may be fewer than the number
> - * requested. If nr_pages is 0 or negative, returns 0. If no pages
> - * were pinned, returns -errno.
> - */
> -int get_user_pages_fast(unsigned long start, int nr_pages,
> -			unsigned int gup_flags, struct page **pages)
> +static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
> +					unsigned int gup_flags,
> +					struct page **pages)

Usualy function are rename to _old_func_name ie add _ in front. So
here it would become _get_user_pages_fast but i know some people
don't like that as sometimes we endup with ___function_overloaded :)

>  {
>  	unsigned long addr, len, end;
>  	int nr = 0, ret = 0;


> @@ -2435,4 +2449,215 @@ int get_user_pages_fast(unsigned long start, int nr_pages,

[...]

> +/**
> + * pin_user_pages_remote() - pin pages for (typically) use by Direct IO, and
> + * return the pages to the user.

Not a fan of (typically) maybe:
pin_user_pages_remote() - pin pages of a remote process (task != current)

I think here the remote part if more important that DIO. Remote is use by
other thing that DIO.

> + *
> + * Nearly the same as get_user_pages_remote(), except that FOLL_PIN is set. See
> + * get_user_pages_remote() for documentation on the function arguments, because
> + * the arguments here are identical.
> + *
> + * FOLL_PIN means that the pages must be released via put_user_page(). Please
> + * see Documentation/vm/pin_user_pages.rst for details.
> + *
> + * This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
> + * is NOT intended for Case 2 (RDMA: long-term pins).
> + */
> +long pin_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
> +			   unsigned long start, unsigned long nr_pages,
> +			   unsigned int gup_flags, struct page **pages,
> +			   struct vm_area_struct **vmas, int *locked)
> +{
> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> +	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> +		return -EINVAL;
> +
> +	gup_flags |= FOLL_TOUCH | FOLL_REMOTE | FOLL_PIN;
> +
> +	return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,
> +				       locked, gup_flags);
> +}
> +EXPORT_SYMBOL(pin_user_pages_remote);
> +
> +/**
> + * pin_longterm_pages_remote() - pin pages for (typically) use by Direct IO, and
> + * return the pages to the user.

I think you copy pasted this from pin_user_pages_remote() :)

> + *
> + * Nearly the same as get_user_pages_remote(), but note that FOLL_TOUCH is not
> + * set, and FOLL_PIN and FOLL_LONGTERM are set. See get_user_pages_remote() for
> + * documentation on the function arguments, because the arguments here are
> + * identical.
> + *
> + * FOLL_PIN means that the pages must be released via put_user_page(). Please
> + * see Documentation/vm/pin_user_pages.rst for further details.
> + *
> + * FOLL_LONGTERM means that the pages are being pinned for "long term" use,
> + * typically by a non-CPU device, and we cannot be sure that waiting for a
> + * pinned page to become unpin will be effective.
> + *
> + * This is intended for Case 2 (RDMA: long-term pins) in
> + * Documentation/vm/pin_user_pages.rst.
> + */
> +long pin_longterm_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
> +			       unsigned long start, unsigned long nr_pages,
> +			       unsigned int gup_flags, struct page **pages,
> +			       struct vm_area_struct **vmas, int *locked)
> +{
> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> +	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> +		return -EINVAL;
> +
> +	/*
> +	 * FIXME: as noted in the get_user_pages_remote() implementation, it
> +	 * is not yet possible to safely set FOLL_LONGTERM here. FOLL_LONGTERM
> +	 * needs to be set, but for now the best we can do is a "TODO" item.
> +	 */
> +	gup_flags |= FOLL_REMOTE | FOLL_PIN;

Wouldn't it be better to not add pin_longterm_pages_remote() until
it can be properly implemented ?


  reply index

Thread overview: 57+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-03 21:17 [PATCH v2 00/18] mm/gup: track dma-pinned pages: FOLL_PIN, FOLL_LONGTERM John Hubbard
2019-11-03 21:17 ` [PATCH v2 01/18] mm/gup: pass flags arg to __gup_device_* functions John Hubbard
2019-11-04 16:39   ` Jerome Glisse
2019-11-03 21:17 ` [PATCH v2 02/18] mm/gup: factor out duplicate code from four routines John Hubbard
2019-11-04 16:51   ` Jerome Glisse
2019-11-03 21:17 ` [PATCH v2 03/18] goldish_pipe: rename local pin_user_pages() routine John Hubbard
2019-11-04 16:52   ` Jerome Glisse
2019-11-03 21:17 ` [PATCH v2 04/18] media/v4l2-core: set pages dirty upon releasing DMA buffers John Hubbard
2019-11-10 10:10   ` Hans Verkuil
2019-11-11 21:46     ` John Hubbard
2019-11-03 21:18 ` [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL_PIN John Hubbard
2019-11-04 17:33   ` Jerome Glisse [this message]
2019-11-04 19:04     ` John Hubbard
2019-11-04 19:18       ` Jerome Glisse
2019-11-04 19:30         ` John Hubbard
2019-11-04 19:52           ` Jerome Glisse
2019-11-04 20:09             ` John Hubbard
2019-11-04 20:31               ` Jason Gunthorpe
2019-11-04 20:40                 ` John Hubbard
2019-11-04 20:31               ` Jerome Glisse
2019-11-04 20:37                 ` Jason Gunthorpe
2019-11-04 20:57                   ` John Hubbard
2019-11-04 21:15                     ` Jason Gunthorpe
2019-11-04 21:34                       ` John Hubbard
2019-11-04 20:33   ` David Rientjes
2019-11-04 20:48     ` Jerome Glisse
2019-11-05 13:10   ` Mike Rapoport
2019-11-05 19:00     ` John Hubbard
2019-11-07  2:25       ` Ira Weiny
2019-11-07  8:07       ` Mike Rapoport
2019-11-03 21:18 ` [PATCH v2 06/18] goldish_pipe: convert to pin_user_pages() and put_user_page() John Hubbard
2019-11-03 21:18 ` [PATCH v2 07/18] infiniband: set FOLL_PIN, FOLL_LONGTERM via pin_longterm_pages*() John Hubbard
2019-11-04 20:33   ` Jason Gunthorpe
2019-11-04 20:48     ` John Hubbard
2019-11-04 20:57       ` Jason Gunthorpe
2019-11-04 22:03         ` John Hubbard
2019-11-05  2:32           ` Jason Gunthorpe
2019-11-07  2:26         ` Ira Weiny
2019-11-03 21:18 ` [PATCH v2 08/18] mm/process_vm_access: set FOLL_PIN via pin_user_pages_remote() John Hubbard
2019-11-04 17:41   ` Jerome Glisse
2019-11-03 21:18 ` [PATCH v2 09/18] drm/via: set FOLL_PIN via pin_user_pages_fast() John Hubbard
2019-11-04 17:44   ` Jerome Glisse
2019-11-04 18:22     ` John Hubbard
2019-11-03 21:18 ` [PATCH v2 10/18] fs/io_uring: set FOLL_PIN via pin_user_pages() John Hubbard
2019-11-03 21:18 ` [PATCH v2 11/18] net/xdp: " John Hubbard
2019-11-03 21:18 ` [PATCH v2 12/18] mm/gup: track FOLL_PIN pages John Hubbard
2019-11-04 18:52   ` Jerome Glisse
2019-11-04 22:49     ` John Hubbard
2019-11-04 23:49       ` Jerome Glisse
2019-11-05  0:18         ` John Hubbard
2019-11-03 21:18 ` [PATCH v2 13/18] media/v4l2-core: pin_longterm_pages (FOLL_PIN) and put_user_page() conversion John Hubbard
2019-11-10 10:11   ` Hans Verkuil
2019-11-03 21:18 ` [PATCH v2 14/18] vfio, mm: " John Hubbard
2019-11-03 21:18 ` [PATCH v2 15/18] powerpc: book3s64: convert to pin_longterm_pages() and put_user_page() John Hubbard
2019-11-03 21:18 ` [PATCH v2 16/18] mm/gup_benchmark: support pin_user_pages() and related calls John Hubbard
2019-11-03 21:18 ` [PATCH v2 17/18] selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN coverage John Hubbard
2019-11-03 21:18 ` [PATCH v2 18/18] mm/gup: remove support for gup(FOLL_LONGTERM) John Hubbard

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