Re: [PATCH 05/19] mm/gup: introduce pin_user_pages*() and FOLL_PIN

On Wed, Oct 30, 2019 at 03:49:16PM -0700, 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 basically documented. (An upcoming patch provides more extensive documentation.) The second set (pin_longterm*) also sets FOLL_LONGTERM:

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().

Signed-off-by: John Hubbard jhubbard@nvidia.com

include/linux/mm.h | 53 ++++++++- mm/gup.c | 284 +++++++++++++++++++++++++++++++++++++++++---- 2 files changed, 311 insertions(+), 26 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h index cc292273e6ba..62c838a3e6c7 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1526,9 +1526,23 @@ long get_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); +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);
  

+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);
  

long get_user_pages(unsigned long start, unsigned long nr_pages, unsigned int gup_flags, struct page **pages, struct vm_area_struct **vmas); +long pin_user_pages(unsigned long start, unsigned long nr_pages,

•     unsigned int gup_flags, struct page **pages,
  

•     struct vm_area_struct **vmas);
  

+long pin_longterm_pages(unsigned long start, unsigned long nr_pages,

• 	unsigned int gup_flags, struct page **pages,
  

• 	struct vm_area_struct **vmas);
  

long get_user_pages_locked(unsigned long start, unsigned long nr_pages, unsigned int gup_flags, struct page **pages, int *locked); long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages, @@ -1536,6 +1550,10 @@ long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages, int get_user_pages_fast(unsigned long start, int nr_pages, unsigned int gup_flags, struct page **pages); +int pin_user_pages_fast(unsigned long start, int nr_pages,

• 	unsigned int gup_flags, struct page **pages);
  

+int pin_longterm_pages_fast(unsigned long start, int nr_pages,

• 	    unsigned int gup_flags, struct page **pages);
  

int account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc); int __account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc, @@ -2594,13 +2612,15 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address, #define FOLL_ANON 0x8000 /* don't do file mappings */ #define FOLL_LONGTERM 0x10000 /* mapping lifetime is indefinite: see below */ #define FOLL_SPLIT_PMD 0x20000 /* split huge pmd before returning */ +#define FOLL_PIN 0x40000 /* pages must be released via put_user_page() */ /*

• • NOTE on FOLL_LONGTERM:

• • FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each
• • other. Here is what they mean, and how to use them:
  • FOLL_LONGTERM indicates that the page will be held for an indefinite time

• • period _often_ under userspace control. This is contrasted with
• • iov_iter_get_pages() where usages which are transient.

• • period _often_ under userspace control. This is in contrast to
• • iov_iter_get_pages(), where usages which are transient.
  • FIXME: For pages which are part of a filesystem, mappings are subject to the
  • lifetime enforced by the filesystem and we need guarantees that longterm

@@ -2615,11 +2635,32 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,

• Currently only get_user_pages() and get_user_pages_fast() support this flag
• and calls to get_user_pages_[un]locked are specifically not allowed. This
• is due to an incompatibility with the FS DAX check and

• • FAULT_FLAG_ALLOW_RETRY

• • FAULT_FLAG_ALLOW_RETRY.

• • In the CMA case: longterm pins in a CMA region would unnecessarily fragment
• • that region. And so CMA attempts to migrate the page before pinning when

• • In the CMA case: long term pins in a CMA region would unnecessarily fragment
• • that region. And so, CMA attempts to migrate the page before pinning, when
  • FOLL_LONGTERM is specified.
• • FOLL_PIN indicates that a special kind of tracking (not just page->_refcount,
• • but an additional pin counting system) will be invoked. This is intended for
• • anything that gets a page reference and then touches page data (for example,
• • Direct IO). This lets the filesystem know that some non-file-system entity is
• • potentially changing the pages' data. In contrast to FOLL_GET (whose pages
• • are released via put_page()), FOLL_PIN pages must be released, ultimately, by
• • a call to put_user_page().
• • FOLL_PIN is similar to FOLL_GET: both of these pin pages. They use different
• • and separate refcounting mechanisms, however, and that means that each has
• • its own acquire and release mechanisms:

• • FOLL_GET: get_user_pages*() to acquire, and put_page() to release.
    

• • FOLL_PIN: pin_user_pages*() or pin_longterm_pages*() to acquire, and
    

• •           put_user_pages to release.
    

• • FOLL_PIN and FOLL_GET are mutually exclusive.

You mean the flags are mutually exclusive for any single call, correct? Because my first thought was that you meant that a page which was pin'ed can't be "got". Which I don't think is true or necessary...

• • Please see Documentation/vm/pin_user_pages.rst for more information.

NIT: I think we should include this file as part of this patch...

*/ static inline int vm_fault_to_errno(vm_fault_t vm_fault, int foll_flags) diff --git a/mm/gup.c b/mm/gup.c index 8fb0d9cdfaf5..8694bc7b3df3 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -179,6 +179,10 @@ static struct page *follow_page_pte(struct vm_area_struct *vma, spinlock_t *ptl; pte_t *ptep, pte;

• /* FOLL_GET and FOLL_PIN are mutually exclusive. */
• if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==

• 	 (FOLL_PIN | FOLL_GET)))
  

• return ERR_PTR(-EINVAL);
  

retry: if (unlikely(pmd_bad(*pmd))) return no_page_table(vma, flags); @@ -790,7 +794,7 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm, start = untagged_addr(start);

• VM_BUG_ON(!!pages != !!(gup_flags & FOLL_GET));

• VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));

/* * If FOLL_FORCE is set then do not force a full fault as the hinting @@ -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;

pages_done = 0; @@ -1133,6 +1146,12 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,

• is written to, set_page_dirty (or set_page_dirty_lock, as appropriate) must
• be called after the page is finished with, and before put_page is called.

• • A note on gup_flags: FOLL_PIN must only be set internally by the
• • pin_user_page*() and pin_longterm_*() APIs, never directly by the caller.
• • That's in order to help avoid mismatches when releasing pages:
• • get_user_pages*() pages must be released via put_page(), while
• • pin_user_pages*() pages must be released via put_user_page().
• • get_user_pages is typically used for fewer-copy IO operations, to get a
  • handle on the memory by some means other than accesses via the user virtual
  • addresses. The pages may be submitted for DMA to devices or accessed via

@@ -1151,6 +1170,14 @@ long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm, unsigned int gup_flags, struct page **pages, struct vm_area_struct **vmas, int *locked) {

• /*

• * As detailed above, FOLL_PIN must only be set internally by the
  

• * pin_user_page*() and pin_longterm_*() APIs, never directly by the
  

• * caller, so enforce that with an assertion:
  

• */
  

• if (WARN_ON_ONCE(gup_flags & FOLL_PIN))

• return -EINVAL;
  

• /*
  • FIXME: Current FOLL_LONGTERM behavior is incompatible with
  • FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on

@@ -1603,11 +1630,25 @@ static __always_inline long __gup_longterm_locked(struct task_struct *tsk,

• and mm being operated on are the current task's and don't allow
• passing of a locked parameter. We also obviously don't pass
• FOLL_REMOTE in here.

• • A note on gup_flags: FOLL_PIN should only be set internally by the
• • pin_user_page*() and pin_longterm_*() APIs, never directly by the caller.
• • That's in order to help avoid mismatches when releasing pages:
• • get_user_pages*() pages must be released via put_page(), while
• • pin_user_pages*() pages must be released via put_user_page().

Rather than put this here should we put it next to the definition of FOLL_PIN? Because now we have this text 2x... :-/

*/ long get_user_pages(unsigned long start, unsigned long nr_pages, unsigned int gup_flags, struct page **pages, struct vm_area_struct **vmas) {

• /*

• * As detailed above, FOLL_PIN must only be set internally by the
  

• * pin_user_page*() and pin_longterm_*() APIs, never directly by the
  

• * caller, so enforce that with an assertion:
  

• */
  

• if (WARN_ON_ONCE(gup_flags & FOLL_PIN))

• return -EINVAL;
  

• return __gup_longterm_locked(current, current->mm, start, nr_pages, pages, vmas, gup_flags | FOLL_TOUCH);

} @@ -2366,24 +2407,9 @@ 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)
  

{ unsigned long addr, len, end; int nr = 0, ret = 0; @@ -2428,4 +2454,222 @@ int get_user_pages_fast(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().
• • A note on gup_flags: FOLL_PIN must only be set internally by the
• • pin_user_page*() and pin_longterm_*() APIs, never directly by the caller.
• • That's in order to help avoid mismatches when releasing pages:
• • get_user_pages*() pages must be released via put_page(), while
• • pin_user_pages*() pages must be released via put_user_page().
• • 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)
  

+{

• /*

• * As detailed above, FOLL_PIN must only be set internally by the
  

• * pin_user_page*() and pin_longterm_*() APIs, never directly by the
  

• * caller, so enforce that:
  

• */
  

• if (WARN_ON_ONCE(gup_flags & FOLL_PIN))

• return -EINVAL;
  

• return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);

+} EXPORT_SYMBOL_GPL(get_user_pages_fast);

+/**

• • pin_user_pages_fast() - pin user pages in memory without taking locks
• • Nearly the same as get_user_pages_fast(), except that FOLL_PIN is set. See
• • get_user_pages_fast() 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.
• • 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).
• */

+int pin_user_pages_fast(unsigned long start, int nr_pages,

• 	unsigned int gup_flags, struct page **pages)
  

+{

• /* FOLL_GET and FOLL_PIN are mutually exclusive. */
• if (WARN_ON_ONCE(gup_flags & FOLL_GET))

• return -EINVAL;
  

• gup_flags |= FOLL_PIN;
• return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);

+} +EXPORT_SYMBOL_GPL(pin_user_pages_fast);

+/**

• • pin_longterm_pages_fast() - pin user pages in memory without taking locks
• • Nearly the same as get_user_pages_fast(), except that FOLL_PIN and
• • FOLL_LONGTERM are set. See get_user_pages_fast() 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) of the FOLL_PIN
• • documentation.
• */

+int pin_longterm_pages_fast(unsigned long start, int nr_pages,

• 	    unsigned int gup_flags, struct page **pages)
  

+{

• /* FOLL_GET and FOLL_PIN are mutually exclusive. */
• if (WARN_ON_ONCE(gup_flags & FOLL_GET))

• return -EINVAL;
  

• gup_flags |= (FOLL_PIN | FOLL_LONGTERM);
• return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);

+} +EXPORT_SYMBOL_GPL(pin_longterm_pages_fast);

+/**

• • pin_user_pages_remote() - pin pages for (typically) use by Direct IO, and
• • return the pages to the user.
• • 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.
• • 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.
  

• */
  

Wait? Why can't we set FOLL_LONGTERM here? pin_* are new calls which are not used yet right?

You set it in the other new pin_* functions?

Ira

• gup_flags |= FOLL_REMOTE | FOLL_PIN;
• return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,

• 		       locked, gup_flags);
  

+} +EXPORT_SYMBOL(pin_longterm_pages_remote);

+/**

• • pin_user_pages() - pin user pages in memory for use by other devices
• • Nearly the same as get_user_pages(), except that FOLL_TOUCH is not set, and
• • FOLL_PIN is set.
• • 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(unsigned long start, unsigned long nr_pages,

•     unsigned int gup_flags, struct page **pages,
  

•     struct vm_area_struct **vmas)
  

+{

• /* FOLL_GET and FOLL_PIN are mutually exclusive. */
• if (WARN_ON_ONCE(gup_flags & FOLL_GET))

• return -EINVAL;
  

• gup_flags |= FOLL_PIN;
• return __gup_longterm_locked(current, current->mm, start, nr_pages,

• 		     pages, vmas, gup_flags);
  

+} +EXPORT_SYMBOL(pin_user_pages);

+/**

• • pin_longterm_pages() - pin user pages in memory for long-term use (RDMA,
• • typically)
• • Nearly the same as get_user_pages(), except that FOLL_PIN and FOLL_LONGTERM
• • are set. See get_user_pages_fast() 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(unsigned long start, unsigned long nr_pages,

• 	unsigned int gup_flags, struct page **pages,
  

• 	struct vm_area_struct **vmas)
  

+{

• /* FOLL_GET and FOLL_PIN are mutually exclusive. */
• if (WARN_ON_ONCE(gup_flags & FOLL_GET))

• return -EINVAL;
  

• gup_flags |= FOLL_PIN | FOLL_LONGTERM;
• return __gup_longterm_locked(current, current->mm, start, nr_pages,

• 		     pages, vmas, gup_flags);
  

+}

+EXPORT_SYMBOL(pin_longterm_pages);

2.23.0