On Sat, Mar 4, 2023 at 10:51 PM Jiaqi Yan <jiaqiyan@xxxxxxxxxx> 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@xxxxxxxxxx>
Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
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
>
