Re: [PATCH v3 1/2] mm/swapfile: fix possible data races of inuse_pages

Huang, Ying
2022-06-27T01:30:14Z

Miaohe Lin <linmiaohe@huawei.com> writes:

> si->inuse_pages could still be accessed concurrently now. The plain reads
> outside si->lock critical section, i.e. swap_show and si_swapinfo, which
> results in data races. READ_ONCE and WRITE_ONCE is used to fix such data
> races. Note these data races should be ok because they're just used for
> showing swap info.
>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>

Reviewed-by: "Huang, Ying" <ying.huang@intel.com>

Thanks!

Best Regards,
Huang, Ying

> ---
> mm/swapfile.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index edc3420d30e7..5c8681a3f1d9 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -695,7 +695,7 @@ static void swap_range_alloc(struct swap_info_struct *si, unsigned long offset,
> si->lowest_bit += nr_entries;
> if (end == si->highest_bit)
> WRITE_ONCE(si->highest_bit, si->highest_bit - nr_entries);
> - si->inuse_pages += nr_entries;
> + WRITE_ONCE(si->inuse_pages, si->inuse_pages + nr_entries);
> if (si->inuse_pages == si->pages) {
> si->lowest_bit = si->max;
> si->highest_bit = 0;
> @@ -732,7 +732,7 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
> add_to_avail_list(si);
> }
> atomic_long_add(nr_entries, &nr_swap_pages);
> - si->inuse_pages -= nr_entries;
> + WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);
> if (si->flags & SWP_BLKDEV)
> swap_slot_free_notify =
> si->bdev->bd_disk->fops->swap_slot_free_notify;
> @@ -2641,7 +2641,7 @@ static int swap_show(struct seq_file *swap, void *v)
> }
>
> bytes = si->pages << (PAGE_SHIFT - 10);
> - inuse = si->inuse_pages << (PAGE_SHIFT - 10);
> + inuse = READ_ONCE(si->inuse_pages) << (PAGE_SHIFT - 10);
>
> file = si->swap_file;
> len = seq_file_path(swap, file, " \t\n\\");
> @@ -3260,7 +3260,7 @@ void si_swapinfo(struct sysinfo *val)
> struct swap_info_struct *si = swap_info[type];
>
> if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK))
> - nr_to_be_unused += si->inuse_pages;
> + nr_to_be_unused += READ_ONCE(si->inuse_pages);
> }
> val->freeswap = atomic_long_read(&nr_swap_pages) + nr_to_be_unused;
> val->totalswap = total_swap_pages + nr_to_be_unused;