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@xxxxxxx>
> Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
Few nitpick belows, nonetheless:
Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> ---
> 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 ?
