Commit c28b1fc7 authored by Jason Gunthorpe's avatar Jason Gunthorpe Committed by Linus Torvalds
Browse files

mm/gup: reorganize internal_get_user_pages_fast() 

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: default avatarJason Gunthorpe <jgg@nvidia.com>
Reviewed-by: default avatarJan Kara <jack@suse.cz>
Reviewed-by: default avatarJohn Hubbard <jhubbard@nvidia.com>
Reviewed-by: default avatarPeter Xu <peterx@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent d0de8241

mm/gup.c

+54 −45
Original line number Diff line number Diff line
@@ -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 addr, len, end;

	unsigned long flags;

	int nr_pinned = 0, ret = 0;

	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 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;

	}

	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 */

	/* 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);



		/* Have to be a bit careful with return values */

		if (nr_pinned > 0) {

			if (ret < 0)

				ret = nr_pinned;

			else

				ret += 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 + nr_pinned;

}



	return ret;

}

/**

 * get_user_pages_fast_only() - pin user pages in memory

 * @start:      starting user address