linux-kernel - [PATCH 1/2] mm: reorganize internal_get_user_pages

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Date:   Fri, 23 Oct 2020 21:19:19 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     <linux-kernel@...r.kernel.org>
CC:     Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Linux-MM <linux-mm@...ck.org>, Michal Hocko <mhocko@...e.com>,
        Oleg Nesterov <oleg@...hat.com>, Peter Xu <peterx@...hat.com>
Subject: [PATCH 1/2] mm: reorganize internal_get_user_pages_fast()

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 << PAGE_SHIFT needs an unsigned long cast, like nr_pages
 - The handling of ret and nr_pinned can be streamlined a bit

No functional change.

Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
---
 mm/gup.c | 88 +++++++++++++++++++++++++++++---------------------------
 1 file changed, 46 insertions(+), 42 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 102877ed77a4b4..ecbe1639ea2af7 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2671,13 +2671,42 @@ static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
 	return ret;
 }
 
+static unsigned int lockless_pages_from_mm(unsigned long addr,
+					   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(addr, 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(addr, end, gup_flags, pages, &nr_pinned);
+	local_irq_restore(flags);
+	return nr_pinned;
+}
+
 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;
-	unsigned long flags;
-	int nr_pinned = 0, ret = 0;
+	unsigned long len, end;
+	unsigned int nr_pinned;
+	int ret;
 
 	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
 				       FOLL_FORCE | FOLL_PIN | FOLL_GET |
@@ -2691,53 +2720,28 @@ 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)
+	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;
-	}
-
-	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);
+	nr_pinned = lockless_pages_from_mm(start, end, gup_flags, pages);
+	if (nr_pinned == nr_pages || gup_flags & FOLL_FAST_ONLY)
+		return nr_pinned;
 
+	/* Try to get the remaining pages with get_user_pages */
+	start += (unsigned long)nr_pinned << PAGE_SHIFT;
+	pages += nr_pinned;
+	ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned, gup_flags,
+				      pages);
+	if (ret < 0) {
 		/* Have to be a bit careful with return values */
-		if (nr_pinned > 0) {
-			if (ret < 0)
-				ret = nr_pinned;
-			else
-				ret += nr_pinned;
-		}
+		if (nr_pinned)
+			return nr_pinned;
+		return ret;
 	}
-
-	return ret;
+	return ret + nr_pinned;
 }
 /**
  * get_user_pages_fast_only() - pin user pages in memory
-- 
2.28.0