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

[Jiaqi Yan <jiaqiyan@google.com>]

On Fri, Mar 24, 2023 at 2:15 PM Yang Shi <shy828301@gmail.com> wrote:
>
> 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;

I notice the "goto xa_locked" statement can be removed now that the
code between here and xa_locked are all gone.

> >                 }
> >         }
> > -
> > -       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.

Ah, for the same reason as the comment above: "Paired with smp_mb() in
do_dentry_open() to ensure i_writecount is up to date and the update
to nr_thps is visible."?
In that case, let me add the smp_mb() like this:
    smp_mb();
    if (!is_shmem && result == SCAN_COPY_MC) {...}

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