Re: [PATCH v10 3/3] mm/khugepaged: recover from poisoned file-backed memory

[Yang Shi <shy828301@gmail.com>]

On Sat, Mar 4, 2023 at 10:51 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 until both scanning and
>   copying succeeded
> - for non-SHMEM file, roll back filemap_nr_thps_inc if scan succeeded but
>   copying failed
>
> Tested manually:
> 0. Enable khugepaged on system under test. Mount tmpfs at /mnt/ramdisk.
> 1. Start a two-thread application. Each thread allocates a chunk of
>    non-huge memory buffer from /mnt/ramdisk.
> 2. Pick 4 random buffer address (2 in each thread) and inject
>    uncorrectable memory errors at physical addresses.
> 3. Signal both threads to make their memory buffer collapsible, i.e.
>    calling madvise(MADV_HUGEPAGE).
> 4. Wait and then check kernel log: khugepaged is able to recover from
>    poisoned pages by skipping them.
> 5. Signal both threads to inspect their buffer contents and make sure no
>    data corruption.
>
> Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>

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

Just a nit below:

> ---
>  mm/khugepaged.c | 78 ++++++++++++++++++++++++++++++-------------------
>  1 file changed, 48 insertions(+), 30 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index c3c217f6ebc6e..3ea2aa55c2c52 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1890,6 +1890,9 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>  {
>         struct address_space *mapping = file->f_mapping;
>         struct page *hpage;
> +       struct page *page;
> +       struct page *tmp;
> +       struct folio *folio;
>         pgoff_t index = 0, end = start + HPAGE_PMD_NR;
>         LIST_HEAD(pagelist);
>         XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
> @@ -1934,8 +1937,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>
>         xas_set(&xas, start);
>         for (index = start; index < end; index++) {
> -               struct page *page = xas_next(&xas);
> -               struct folio *folio;
> +               page = xas_next(&xas);
>
>                 VM_BUG_ON(index != xas.xa_index);
>                 if (is_shmem) {
> @@ -2117,10 +2119,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>         }
>         nr = thp_nr_pages(hpage);
>
> -       if (is_shmem)
> -               __mod_lruvec_page_state(hpage, NR_SHMEM_THPS, nr);
> -       else {
> -               __mod_lruvec_page_state(hpage, NR_FILE_THPS, nr);
> +       if (!is_shmem) {
>                 filemap_nr_thps_inc(mapping);
>                 /*
>                  * Paired with smp_mb() in do_dentry_open() to ensure
> @@ -2131,21 +2130,10 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>                 smp_mb();
>                 if (inode_is_open_for_write(mapping->host)) {
>                         result = SCAN_FAIL;
> -                       __mod_lruvec_page_state(hpage, NR_FILE_THPS, -nr);
>                         filemap_nr_thps_dec(mapping);
>                         goto xa_locked;
>                 }
>         }
> -
> -       if (nr_none) {
> -               __mod_lruvec_page_state(hpage, NR_FILE_PAGES, nr_none);
> -               /* nr_none is always 0 for non-shmem. */
> -               __mod_lruvec_page_state(hpage, 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, hpage);
>  xa_locked:
>         xas_unlock_irq(&xas);
>  xa_unlocked:
> @@ -2158,21 +2146,35 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>         try_to_unmap_flush();
>
>         if (result == SCAN_SUCCEED) {
> -               struct page *page, *tmp;
> -               struct folio *folio;
> -
>                 /*
>                  * 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(hpage + (index % HPAGE_PMD_NR));
>                                 index++;
>                         }
> -                       copy_highpage(hpage + (page->index % HPAGE_PMD_NR),
> -                                     page);
> +                       if (copy_mc_highpage(hpage + (page->index % HPAGE_PMD_NR),
> +                                            page) > 0) {
> +                               result = SCAN_COPY_MC;
> +                               break;
> +                       }
> +                       index++;
> +               }
> +               while (result == SCAN_SUCCEED && index < end) {
> +                       clear_highpage(hpage + (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);
> @@ -2180,12 +2182,23 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>                         ClearPageUnevictable(page);
>                         unlock_page(page);
>                         put_page(page);
> -                       index++;
>                 }
> -               while (index < end) {
> -                       clear_highpage(hpage + (index % HPAGE_PMD_NR));
> -                       index++;
> +
> +               xas_lock_irq(&xas);
> +               if (is_shmem)
> +                       __mod_lruvec_page_state(hpage, NR_SHMEM_THPS, nr);
> +               else
> +                       __mod_lruvec_page_state(hpage, NR_FILE_THPS, nr);
> +
> +               if (nr_none) {
> +                       __mod_lruvec_page_state(hpage, NR_FILE_PAGES, nr_none);
> +                       /* nr_none is always 0 for non-shmem. */
> +                       __mod_lruvec_page_state(hpage, 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, hpage);
> +               xas_unlock_irq(&xas);
>
>                 folio = page_folio(hpage);
>                 folio_mark_uptodate(folio);
> @@ -2203,8 +2216,6 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>                 unlock_page(hpage);
>                 hpage = NULL;
>         } else {
> -               struct page *page;
> -
>                 /* Something went wrong: roll back page cache changes */
>                 xas_lock_irq(&xas);
>                 if (nr_none) {
> @@ -2238,6 +2249,13 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>                         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);

We may need a memory barrier here. But missing the memory barrier is
not a fatal issue either, the worst case is unnecessary truncate from
open path if it sees obsolete nr_thps counter. And it may be better to
handle it in a follow up patch by moving smp_mb() into
filemap_nr_thp_xxx functions.

> +
>                 xas_unlock_irq(&xas);
>
>                 hpage->mapping = NULL;
> --
> 2.40.0.rc0.216.gc4246ad0f0-goog
>