Re: [PATCH v3 2/2] mm: khugepaged: recover from poisoned file-backed memory

[Yang Shi <shy828301@gmail.com>]

On Mon, May 23, 2022 at 7:54 PM Jiaqi Yan <jiaqiyan@google.com> wrote:
>
> Make collapse_file roll back when copying pages failed.
> More concretely:
> * extract copying operations into a separate loop
> * postpone the updates for nr_none until both scanning and
>   copying succeeded
> * postpone joining small xarray entries until both scanning and
>   copying succeeded
> * postpone the update operations to NR_XXX_THPS
> * for non-SHMEM file, roll back filemap_nr_thps_inc if scan
>   succeeded but copying failed
>
> Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>

Reviewed-by: Yang Shi <shy828301@gmail.com>

Just a nit below.

> ---
>  mm/khugepaged.c | 77 ++++++++++++++++++++++++++++++-------------------
>  1 file changed, 48 insertions(+), 29 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 1b08e31ba081a..98976622ee7c5 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1716,7 +1716,7 @@ static void collapse_file(struct mm_struct *mm,
>  {
>         struct address_space *mapping = file->f_mapping;
>         gfp_t gfp;
> -       struct page *new_page;
> +       struct page *new_page, *page, *tmp;
>         pgoff_t index, end = start + HPAGE_PMD_NR;
>         LIST_HEAD(pagelist);
>         XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
> @@ -1772,7 +1772,7 @@ static void collapse_file(struct mm_struct *mm,
>
>         xas_set(&xas, start);
>         for (index = start; index < end; index++) {
> -               struct page *page = xas_next(&xas);
> +               page = xas_next(&xas);
>
>                 VM_BUG_ON(index != xas.xa_index);
>                 if (is_shmem) {
> @@ -1944,10 +1944,7 @@ static void collapse_file(struct mm_struct *mm,
>         }
>         nr = thp_nr_pages(new_page);
>
> -       if (is_shmem)
> -               __mod_lruvec_page_state(new_page, NR_SHMEM_THPS, nr);
> -       else {
> -               __mod_lruvec_page_state(new_page, NR_FILE_THPS, nr);
> +       if (!is_shmem) {
>                 filemap_nr_thps_inc(mapping);
>                 /*
>                  * Paired with smp_mb() in do_dentry_open() to ensure
> @@ -1958,40 +1955,44 @@ static void collapse_file(struct mm_struct *mm,
>                 smp_mb();
>                 if (inode_is_open_for_write(mapping->host)) {
>                         result = SCAN_FAIL;
> -                       __mod_lruvec_page_state(new_page, NR_FILE_THPS, -nr);
>                         filemap_nr_thps_dec(mapping);
>                         goto xa_locked;
>                 }
>         }
>
> -       if (nr_none) {
> -               __mod_lruvec_page_state(new_page, NR_FILE_PAGES, nr_none);
> -               if (is_shmem)
> -                       __mod_lruvec_page_state(new_page, NR_SHMEM, nr_none);
> -       }
> -
> -       /* Join all the small entries into a single multi-index entry */
> -       xas_set_order(&xas, start, HPAGE_PMD_ORDER);
> -       xas_store(&xas, new_page);
>  xa_locked:
>         xas_unlock_irq(&xas);
>  xa_unlocked:
>
>         if (result == SCAN_SUCCEED) {
> -               struct page *page, *tmp;
> -
>                 /*
>                  * Replacing old pages with new one has succeeded, now we
> -                * need to copy the content and free the old pages.
> +                * attempt to copy the contents.
>                  */
>                 index = start;
> -               list_for_each_entry_safe(page, tmp, &pagelist, lru) {
> +               list_for_each_entry(page, &pagelist, lru) {
>                         while (index < page->index) {
>                                 clear_highpage(new_page + (index % HPAGE_PMD_NR));
>                                 index++;
>                         }
> -                       copy_highpage(new_page + (page->index % HPAGE_PMD_NR),
> -                                       page);
> +                       if (copy_highpage_mc(new_page + (page->index % HPAGE_PMD_NR), page)) {
> +                               result = SCAN_COPY_MC;
> +                               break;
> +                       }
> +                       index++;
> +               }
> +               while (result == SCAN_SUCCEED && index < end) {
> +                       clear_highpage(new_page + (page->index % HPAGE_PMD_NR));
> +                       index++;
> +               }
> +       }
> +
> +       if (result == SCAN_SUCCEED) {
> +               /*
> +                * Copying old pages to huge one has succeeded, now we
> +                * need to free the old pages.
> +                */
> +               list_for_each_entry_safe(page, tmp, &pagelist, lru) {
>                         list_del(&page->lru);
>                         page->mapping = NULL;
>                         page_ref_unfreeze(page, 1);
> @@ -1999,12 +2000,23 @@ static void collapse_file(struct mm_struct *mm,
>                         ClearPageUnevictable(page);
>                         unlock_page(page);
>                         put_page(page);
> -                       index++;
>                 }
> -               while (index < end) {
> -                       clear_highpage(new_page + (index % HPAGE_PMD_NR));
> -                       index++;
> +
> +               xas_lock_irq(&xas);
> +               if (is_shmem)
> +                       __mod_lruvec_page_state(new_page, NR_SHMEM_THPS, nr);
> +               else
> +                       __mod_lruvec_page_state(new_page, NR_FILE_THPS, nr);
> +
> +               if (nr_none) {
> +                       __mod_lruvec_page_state(new_page, NR_FILE_PAGES, nr_none);
> +                       if (is_shmem)
> +                               __mod_lruvec_page_state(new_page, NR_SHMEM, nr_none);
>                 }
> +               /* Join all the small entries into a single multi-index entry. */
> +               xas_set_order(&xas, start, HPAGE_PMD_ORDER);
> +               xas_store(&xas, new_page);
> +               xas_unlock_irq(&xas);
>
>                 SetPageUptodate(new_page);
>                 page_ref_add(new_page, HPAGE_PMD_NR - 1);
> @@ -2020,9 +2032,9 @@ static void collapse_file(struct mm_struct *mm,
>
>                 khugepaged_pages_collapsed++;
>         } else {
> -               struct page *page;
> -
> -               /* Something went wrong: roll back page cache changes */
> +               /*
> +                * Something went wrong: roll back page cache changes
> +                */

Changing from one line to multiple lines seems pointless.

>                 xas_lock_irq(&xas);
>                 mapping->nrpages -= nr_none;
>
> @@ -2055,6 +2067,13 @@ static void collapse_file(struct mm_struct *mm,
>                         xas_lock_irq(&xas);
>                 }
>                 VM_BUG_ON(nr_none);
> +               /*
> +                * Undo the updates of filemap_nr_thps_inc for non-SHMEM file only.
> +                * This undo is not needed unless failure is due to SCAN_COPY_MC.
> +                */
> +               if (!is_shmem && result == SCAN_COPY_MC)
> +                       filemap_nr_thps_dec(mapping);
> +
>                 xas_unlock_irq(&xas);
>
>                 new_page->mapping = NULL;
> --
> 2.36.1.124.g0e6072fb45-goog
>