Re: [PATCH] mm/gup: Remove the restriction on locked with FOLL_LONGTERM

John Hubbard <jhubbard@xxxxxxxxxx> · Wed, 16 Nov 2022 14:11:04 -0800

On 11/16/22 12:07, Jason Gunthorpe wrote:
This restriction was created because FOLL_LONGTERM used to scan the vma
list, so it could not tolerate becoming unlocked. That was fixed in commit
52650c8b466b ("mm/gup: remove the vma allocation from
gup_longterm_locked()") and the restriction on !vma was removed.

However, the locked restriction remained, even though it isn't necessary
anymore.

Adjust __gup_longterm_locked() so it can handle the mmap_read_lock()
becoming unlocked while it is looping for migration. Migration does not
require the mmap_read_sem because it is only handling struct pages.  If we
had to unlock then ensure the whole thing returns unlocked.

Remove __get_user_pages_remote() and __gup_longterm_unlocked(). These
cases can now just directly call other functions.

Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
---
  mm/gup.c | 109 ++++++++++++++-----------------------------------------
  1 file changed, 27 insertions(+), 82 deletions(-)

Looks nice.

Even after this cleanup, gup.c is still a bit of a ball of string, what
with things like __gup_longterm_locked() actually handling
!FOLL_LONGTERM, and a bunch of other vestigial stuff, but this takes us
in the right direction.


Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>

thanks,
--
John Hubbard
NVIDIA

diff --git a/mm/gup.c b/mm/gup.c
index fe195d47de74a7..91251291698a08 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2105,14 +2105,19 @@ static long __gup_longterm_locked(struct mm_struct *mm,
  				  unsigned long nr_pages,
  				  struct page **pages,
  				  struct vm_area_struct **vmas,
+				  int *locked,
  				  unsigned int gup_flags)
  {
+	bool must_unlock = false;
  	unsigned int flags;
  	long rc, nr_pinned_pages;
  
+	if (locked && WARN_ON_ONCE(!*locked))
+		return -EINVAL;
+
  	if (!(gup_flags & FOLL_LONGTERM))
  		return __get_user_pages_locked(mm, start, nr_pages, pages, vmas,
-					       NULL, gup_flags);
+					       locked, gup_flags);
  
  	/*
  	 * If we get to this point then FOLL_LONGTERM is set, and FOLL_LONGTERM
@@ -2126,8 +2131,13 @@ static long __gup_longterm_locked(struct mm_struct *mm,
  		return -EINVAL;
  	flags = memalloc_pin_save();
  	do {
+		if (locked && !*locked) {
+			mmap_read_lock(mm);
+			must_unlock = true;
+			*locked = 1;
+		}
  		nr_pinned_pages = __get_user_pages_locked(mm, start, nr_pages,
-							  pages, vmas, NULL,
+							  pages, vmas, locked,
  							  gup_flags);
  		if (nr_pinned_pages <= 0) {
  			rc = nr_pinned_pages;
@@ -2137,6 +2147,10 @@ static long __gup_longterm_locked(struct mm_struct *mm,
  	} while (rc == -EAGAIN);
  	memalloc_pin_restore(flags);
  
+	if (locked && *locked && must_unlock) {
+		mmap_read_unlock(mm);
+		*locked = 0;
+	}
  	return rc ? rc : nr_pinned_pages;
  }
  
@@ -2160,35 +2174,6 @@ static bool is_valid_gup_flags(unsigned int gup_flags)
  }
  
  #ifdef CONFIG_MMU
-static long __get_user_pages_remote(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)
-{
-	/*
-	 * Parts of FOLL_LONGTERM behavior are incompatible with
-	 * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
-	 * 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 (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(mm, start, nr_pages, pages,
-					     vmas, gup_flags | FOLL_TOUCH |
-					     FOLL_REMOTE);
-	}
-
-	return __get_user_pages_locked(mm, start, nr_pages, pages, vmas,
-				       locked,
-				       gup_flags | FOLL_TOUCH | FOLL_REMOTE);
-}
-
  /**
   * get_user_pages_remote() - pin user pages in memory
   * @mm:		mm_struct of target mm
@@ -2257,8 +2242,8 @@ long get_user_pages_remote(struct mm_struct *mm,
  	if (!is_valid_gup_flags(gup_flags))
  		return -EINVAL;
  
-	return __get_user_pages_remote(mm, start, nr_pages, gup_flags,
-				       pages, vmas, locked);
+	return __gup_longterm_locked(mm, start, nr_pages, pages, vmas, locked,
+				     gup_flags | FOLL_TOUCH | FOLL_REMOTE);
  }
  EXPORT_SYMBOL(get_user_pages_remote);
  
@@ -2270,14 +2255,6 @@ long get_user_pages_remote(struct mm_struct *mm,
  {
  	return 0;
  }
-
-static long __get_user_pages_remote(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)
-{
-	return 0;
-}
  #endif /* !CONFIG_MMU */
  
  /**
@@ -2304,7 +2281,7 @@ long get_user_pages(unsigned long start, unsigned long nr_pages,
  		return -EINVAL;
  
  	return __gup_longterm_locked(current->mm, start, nr_pages,
-				     pages, vmas, gup_flags | FOLL_TOUCH);
+				     pages, vmas, NULL, gup_flags | FOLL_TOUCH);
  }
  EXPORT_SYMBOL(get_user_pages);
  
@@ -2330,18 +2307,9 @@ long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
  	int locked = 1;
  	long ret;
  
-	/*
-	 * FIXME: Current FOLL_LONGTERM behavior is 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.
-	 */
-	if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
-		return -EINVAL;
-
  	mmap_read_lock(mm);
-	ret = __get_user_pages_locked(mm, start, nr_pages, pages, NULL,
-				      &locked, gup_flags | FOLL_TOUCH);
+	ret = __gup_longterm_locked(mm, start, nr_pages, pages, NULL, &locked,
+				    gup_flags | FOLL_TOUCH);
  	if (locked)
  		mmap_read_unlock(mm);
  	return ret;
@@ -2935,29 +2903,6 @@ static bool gup_fast_permitted(unsigned long start, unsigned long end)
  }
  #endif
  
-static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
-				   unsigned int gup_flags, struct page **pages)
-{
-	int ret;
-
-	/*
-	 * FIXME: FOLL_LONGTERM does not work with
-	 * get_user_pages_unlocked() (see comments in that function)
-	 */
-	if (gup_flags & FOLL_LONGTERM) {
-		mmap_read_lock(current->mm);
-		ret = __gup_longterm_locked(current->mm,
-					    start, nr_pages,
-					    pages, NULL, gup_flags);
-		mmap_read_unlock(current->mm);
-	} else {
-		ret = get_user_pages_unlocked(start, nr_pages,
-					      pages, gup_flags);
-	}
-
-	return ret;
-}
-
  static unsigned long lockless_pages_from_mm(unsigned long start,
  					    unsigned long end,
  					    unsigned int gup_flags,
@@ -3041,8 +2986,8 @@ static int internal_get_user_pages_fast(unsigned long start,
  	/* Slow path: try to get the remaining pages with get_user_pages */
  	start += nr_pinned << PAGE_SHIFT;
  	pages += nr_pinned;
-	ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned, gup_flags,
-				      pages);
+	ret = get_user_pages_unlocked(start, nr_pages - nr_pinned, pages,
+				      gup_flags);
  	if (ret < 0) {
  		/*
  		 * The caller has to unpin the pages we already pinned so
@@ -3241,9 +3186,9 @@ long pin_user_pages_remote(struct mm_struct *mm,
  	if (WARN_ON_ONCE(!pages))
  		return -EINVAL;
  
-	gup_flags |= FOLL_PIN;
-	return __get_user_pages_remote(mm, start, nr_pages, gup_flags,
-				       pages, vmas, locked);
+	return __gup_longterm_locked(mm, start, nr_pages, pages, vmas, locked,
+				     gup_flags | FOLL_PIN | FOLL_TOUCH |
+					     FOLL_REMOTE);
  }
  EXPORT_SYMBOL(pin_user_pages_remote);
  
@@ -3277,7 +3222,7 @@ long pin_user_pages(unsigned long start, unsigned long nr_pages,
  
  	gup_flags |= FOLL_PIN;
  	return __gup_longterm_locked(current->mm, start, nr_pages,
-				     pages, vmas, gup_flags);
+				     pages, vmas, NULL, gup_flags);
  }
  EXPORT_SYMBOL(pin_user_pages);
  

base-commit: b1a4dd7dab677fa4bfc4967c99ee45ba4d1d05d6








