Re: [PATCH v2 1/2] mm: reorganize internal_get_user_pages_fast()

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On 10/30/20 7:46 AM, Jason Gunthorpe wrote:
The next patch in this series makes the lockless flow a little more
complex, so move the entire block into a new function and remove a level
of indention. Tidy a bit of cruft:

  - addr is always the same as start, so use start

  - Use the modern check_add_overflow() for computing end = start + len

  - nr_pinned/pages << PAGE_SHIFT needs the LHS to be unsigned long to
    avoid shift overflow, make the variables unsigned long to avoid coding
    casts in both places. nr_pinned was missing its cast

  - The handling of ret and nr_pinned can be streamlined a bit

No functional change.

Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
---
  mm/gup.c | 99 ++++++++++++++++++++++++++++++--------------------------
  1 file changed, 54 insertions(+), 45 deletions(-)

Everything still looks correct.

Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>


thanks,
--
John Hubbard
NVIDIA



diff --git a/mm/gup.c b/mm/gup.c
index 102877ed77a4b4..150cc962c99201 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2671,13 +2671,43 @@ static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
  	return ret;
  }
-static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
+static unsigned long lockless_pages_from_mm(unsigned long start,
+					    unsigned long end,
+					    unsigned int gup_flags,
+					    struct page **pages)
+{
+	unsigned long flags;
+	int nr_pinned = 0;
+
+	if (!IS_ENABLED(CONFIG_HAVE_FAST_GUP) ||
+	    !gup_fast_permitted(start, end))
+		return 0;
+
+	/*
+	 * Disable interrupts. The nested form is used, in order to allow full,
+	 * general purpose use of this routine.
+	 *
+	 * With interrupts disabled, we block page table pages from being freed
+	 * from under us. See struct mmu_table_batch comments in
+	 * include/asm-generic/tlb.h for more details.
+	 *
+	 * We do not adopt an rcu_read_lock() here as we also want to block IPIs
+	 * that come from THPs splitting.
+	 */
+	local_irq_save(flags);
+	gup_pgd_range(start, end, gup_flags, pages, &nr_pinned);
+	local_irq_restore(flags);
+	return nr_pinned;
+}
+
+static int internal_get_user_pages_fast(unsigned long start,
+					unsigned long nr_pages,
  					unsigned int gup_flags,
  					struct page **pages)
  {
-	unsigned long addr, len, end;
-	unsigned long flags;
-	int nr_pinned = 0, ret = 0;
+	unsigned long len, end;
+	unsigned long nr_pinned;
+	int ret;
if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
  				       FOLL_FORCE | FOLL_PIN | FOLL_GET |
@@ -2691,54 +2721,33 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
  		might_lock_read(&current->mm->mmap_lock);
start = untagged_addr(start) & PAGE_MASK;
-	addr = start;
-	len = (unsigned long) nr_pages << PAGE_SHIFT;
-	end = start + len;
-
-	if (end <= start)
+	len = nr_pages << PAGE_SHIFT;
+	if (check_add_overflow(start, len, &end))
  		return 0;
  	if (unlikely(!access_ok((void __user *)start, len)))
  		return -EFAULT;
- /*
-	 * Disable interrupts. The nested form is used, in order to allow
-	 * full, general purpose use of this routine.
-	 *
-	 * With interrupts disabled, we block page table pages from being
-	 * freed from under us. See struct mmu_table_batch comments in
-	 * include/asm-generic/tlb.h for more details.
-	 *
-	 * We do not adopt an rcu_read_lock(.) here as we also want to
-	 * block IPIs that come from THPs splitting.
-	 */
-	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && gup_fast_permitted(start, end)) {
-		unsigned long fast_flags = gup_flags;
-
-		local_irq_save(flags);
-		gup_pgd_range(addr, end, fast_flags, pages, &nr_pinned);
-		local_irq_restore(flags);
-		ret = nr_pinned;
-	}
+	nr_pinned = lockless_pages_from_mm(start, end, gup_flags, pages);
+	if (nr_pinned == nr_pages || gup_flags & FOLL_FAST_ONLY)
+		return nr_pinned;
- if (nr_pinned < nr_pages && !(gup_flags & FOLL_FAST_ONLY)) {
-		/* 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);
-
-		/* Have to be a bit careful with return values */
-		if (nr_pinned > 0) {
-			if (ret < 0)
-				ret = nr_pinned;
-			else
-				ret += nr_pinned;
-		}
+	/* 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);
+	if (ret < 0) {
+		/*
+		 * The caller has to unpin the pages we already pinned so
+		 * returning -errno is not an option
+		 */
+		if (nr_pinned)
+			return nr_pinned;
+		return ret;
  	}
-
-	return ret;
+	return ret + nr_pinned;
  }
+
  /**
   * get_user_pages_fast_only() - pin user pages in memory
   * @start:      starting user address






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