Re: [PATCH] mm: fix special swap entry handling on copy mm

Naoya Horiguchi
2013-08-12T21:37:03Z

Hi Jerome,

On Mon, Aug 12, 2013 at 11:43:24AM -0400, j.glisse@gmail.com wrote:
> From: Jerome Glisse <jglisse@redhat.com>
>
> Prior to this copy_one_pte will never reach the special swap file
> handling code because swap_duplicate will return invalid value.
>
> Note this is not fatal so nothing bad ever happen because of that.
> Reason is that copy_pte_range would break of its loop and call
> add_swap_count_continuation which would see its a special swap
> file and return 0 triggering copy_pte_range to try again. Because
> we try again there is a huge chance that the temporarily special
> migration pte is now again valid and pointing to a new valid page.
>
> This patch just split handling of special swap entry from regular
> one inside copy_one_pte.
>
> (Note i spotted that while reading code i haven't tested my theory.)
>
> Signed-off-by: Jerome Glisse <jglisse@redhat.com>

non_swap_entry() means not only migration entry, but also hwpoison entry,
so it seems to me that simply moving the swap_duplicate() into the
if(!non_swap_entry) block can change the behavior for hwpoison entry.
Would it be nice to add check for such a case?

Thanks,
Naoya Horiguchi

> ---
> mm/memory.c | 26 +++++++++++++-------------
> 1 file changed, 13 insertions(+), 13 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 1ce2e2a..9f907dd 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -833,20 +833,20 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> if (!pte_file(pte)) {
> swp_entry_t entry = pte_to_swp_entry(pte);
>
> - if (swap_duplicate(entry) < 0)
> - return entry.val;
> -
> - /* make sure dst_mm is on swapoff's mmlist. */
> - if (unlikely(list_empty(&dst_mm->mmlist))) {
> - spin_lock(&mmlist_lock);
> - if (list_empty(&dst_mm->mmlist))
> - list_add(&dst_mm->mmlist,
> - &src_mm->mmlist);
> - spin_unlock(&mmlist_lock);
> - }
> - if (likely(!non_swap_entry(entry)))
> + if (likely(!non_swap_entry(entry))) {
> + if (swap_duplicate(entry) < 0)
> + return entry.val;
> +
> + /* make sure dst_mm is on swapoff's mmlist. */
> + if (unlikely(list_empty(&dst_mm->mmlist))) {
> + spin_lock(&mmlist_lock);
> + if (list_empty(&dst_mm->mmlist))
> + list_add(&dst_mm->mmlist,
> + &src_mm->mmlist);
> + spin_unlock(&mmlist_lock);
> + }
> rss[MM_SWAPENTS]++;
> - else if (is_migration_entry(entry)) {
> + } else if (is_migration_entry(entry)) {
> page = migration_entry_to_page(entry);
>
> if (PageAnon(page))
> --
> 1.8.3.1
>
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