[PATCH 5.10 446/717] mm/gup: reorganize internal_get_user_pages_fast()

Greg Kroah-Hartman
2020-12-28T14:49:51Z

From: Jason Gunthorpe <jgg@nvidia.com>

[ Upstream commit c28b1fc70390df32e29991eedd52bd86e7aba080 ]

Patch series "Add a seqcount between gup_fast and copy_page_range()", v4.

As discussed and suggested by Linus use a seqcount to close the small race
between gup_fast and copy_page_range().

Ahmed confirms that raw_write_seqcount_begin() is the correct API to use
in this case and it doesn't trigger any lockdeps.

I was able to test it using two threads, one forking and the other using
ibv_reg_mr() to trigger GUP fast. Modifying copy_page_range() to sleep
made the window large enough to reliably hit to test the logic.

This patch (of 2):

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.

Link: https://lkml.kernel.org/r/0-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com
Link: https://lkml.kernel.org/r/1-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/gup.c | 99 ++++++++++++++++++++++++++++++--------------------------
1 file changed, 54 insertions(+), 45 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 98eb8e6d2609c..c7e24301860ab 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2677,13 +2677,43 @@ 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 |
@@ -2697,54 +2727,33 @@ 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;
- }
-
- 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;
+ nr_pinned = lockless_pages_from_mm(start, end, gup_flags, pages);
+ if (nr_pinned == nr_pages || gup_flags & FOLL_FAST_ONLY)
+ return 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
--
2.27.0