Re: [PATCH V2] mm/gup: Clear the LRU flag of a page before adding to LRU batch

[Barry Song <21cnbao@gmail.com>]

On Sat, Jun 22, 2024 at 6:48 PM <yangge1116@126.com> wrote:
>
> From: yangge <yangge1116@126.com>
>
> If a large number of CMA memory are configured in system (for example, the
> CMA memory accounts for 50% of the system memory), starting a virtual
> virtual machine, it will call pin_user_pages_remote(..., FOLL_LONGTERM,
> ...) to pin memory.  Normally if a page is present and in CMA area,
> pin_user_pages_remote() will migrate the page from CMA area to non-CMA
> area because of FOLL_LONGTERM flag. But the current code will cause the
> migration failure due to unexpected page refcounts, and eventually cause
> the virtual machine fail to start.
>
> If a page is added in LRU batch, its refcount increases one, remove the
> page from LRU batch decreases one. Page migration requires the page is not
> referenced by others except page mapping. Before migrating a page, we
> should try to drain the page from LRU batch in case the page is in it,
> however, folio_test_lru() is not sufficient to tell whether the page is
> in LRU batch or not, if the page is in LRU batch, the migration will fail.
>
> To solve the problem above, we modify the logic of adding to LRU batch.
> Before adding a page to LRU batch, we clear the LRU flag of the page so
> that we can check whether the page is in LRU batch by folio_test_lru(page).
> Seems making the LRU flag of the page invisible a long time is no problem,
> because a new page is allocated from buddy and added to the lru batch,
> its LRU flag is also not visible for a long time.
>
> Cc: <stable@vger.kernel.org>

you have Cced stable, what is the fixes tag?

> Signed-off-by: yangge <yangge1116@126.com>
> ---
>  mm/swap.c | 43 +++++++++++++++++++++++++++++++------------
>  1 file changed, 31 insertions(+), 12 deletions(-)
>
> diff --git a/mm/swap.c b/mm/swap.c
> index dc205bd..9caf6b0 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -211,10 +211,6 @@ static void folio_batch_move_lru(struct folio_batch *fbatch, move_fn_t move_fn)
>         for (i = 0; i < folio_batch_count(fbatch); i++) {
>                 struct folio *folio = fbatch->folios[i];
>
> -               /* block memcg migration while the folio moves between lru */
> -               if (move_fn != lru_add_fn && !folio_test_clear_lru(folio))
> -                       continue;
> -
>                 folio_lruvec_relock_irqsave(folio, &lruvec, &flags);
>                 move_fn(lruvec, folio);
>
> @@ -255,11 +251,16 @@ static void lru_move_tail_fn(struct lruvec *lruvec, struct folio *folio)
>  void folio_rotate_reclaimable(struct folio *folio)
>  {
>         if (!folio_test_locked(folio) && !folio_test_dirty(folio) &&
> -           !folio_test_unevictable(folio) && folio_test_lru(folio)) {
> +           !folio_test_unevictable(folio)) {
>                 struct folio_batch *fbatch;
>                 unsigned long flags;
>
>                 folio_get(folio);
> +               if (!folio_test_clear_lru(folio)) {
> +                       folio_put(folio);
> +                       return;
> +               }
> +
>                 local_lock_irqsave(&lru_rotate.lock, flags);
>                 fbatch = this_cpu_ptr(&lru_rotate.fbatch);
>                 folio_batch_add_and_move(fbatch, folio, lru_move_tail_fn);
> @@ -352,11 +353,15 @@ static void folio_activate_drain(int cpu)
>
>  void folio_activate(struct folio *folio)
>  {
> -       if (folio_test_lru(folio) && !folio_test_active(folio) &&
> -           !folio_test_unevictable(folio)) {
> +       if (!folio_test_active(folio) && !folio_test_unevictable(folio)) {
>                 struct folio_batch *fbatch;
>
>                 folio_get(folio);
> +               if (!folio_test_clear_lru(folio)) {
> +                       folio_put(folio);
> +                       return;
> +               }
> +
>                 local_lock(&cpu_fbatches.lock);
>                 fbatch = this_cpu_ptr(&cpu_fbatches.activate);
>                 folio_batch_add_and_move(fbatch, folio, folio_activate_fn);
> @@ -700,6 +705,11 @@ void deactivate_file_folio(struct folio *folio)
>                 return;
>
>         folio_get(folio);
> +       if (!folio_test_clear_lru(folio)) {
> +               folio_put(folio);
> +               return;
> +       }
> +
>         local_lock(&cpu_fbatches.lock);
>         fbatch = this_cpu_ptr(&cpu_fbatches.lru_deactivate_file);
>         folio_batch_add_and_move(fbatch, folio, lru_deactivate_file_fn);
> @@ -716,11 +726,16 @@ void deactivate_file_folio(struct folio *folio)
>   */
>  void folio_deactivate(struct folio *folio)
>  {
> -       if (folio_test_lru(folio) && !folio_test_unevictable(folio) &&
> -           (folio_test_active(folio) || lru_gen_enabled())) {
> +       if (!folio_test_unevictable(folio) && (folio_test_active(folio) ||
> +           lru_gen_enabled())) {
>                 struct folio_batch *fbatch;
>
>                 folio_get(folio);
> +               if (!folio_test_clear_lru(folio)) {
> +                       folio_put(folio);
> +                       return;
> +               }
> +
>                 local_lock(&cpu_fbatches.lock);
>                 fbatch = this_cpu_ptr(&cpu_fbatches.lru_deactivate);
>                 folio_batch_add_and_move(fbatch, folio, lru_deactivate_fn);
> @@ -737,12 +752,16 @@ void folio_deactivate(struct folio *folio)
>   */
>  void folio_mark_lazyfree(struct folio *folio)
>  {
> -       if (folio_test_lru(folio) && folio_test_anon(folio) &&
> -           folio_test_swapbacked(folio) && !folio_test_swapcache(folio) &&
> -           !folio_test_unevictable(folio)) {
> +       if (folio_test_anon(folio) && folio_test_swapbacked(folio) &&
> +           !folio_test_swapcache(folio) && !folio_test_unevictable(folio)) {
>                 struct folio_batch *fbatch;
>
>                 folio_get(folio);
> +               if (!folio_test_clear_lru(folio)) {
> +                       folio_put(folio);
> +                       return;
> +               }
> +
>                 local_lock(&cpu_fbatches.lock);
>                 fbatch = this_cpu_ptr(&cpu_fbatches.lru_lazyfree);
>                 folio_batch_add_and_move(fbatch, folio, lru_lazyfree_fn);
> --
> 2.7.4
>