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

[Jiaqi Yan <jiaqiyan@google.com>]

On Fri, Mar 24, 2023 at 5:39 PM Hugh Dickins <hughd@google.com> wrote:
>
> On Fri, 24 Mar 2023, Jiaqi Yan wrote:
> > 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:
>
> Acked-by: Hugh Dickins <hughd@google.com>
>
> with a little nit from me below, if you are respinning:
>
> > >
> > > > ---
> > > >  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
>
> That "nr = thp_nr_pages(hpage);" above becomes stranded a long way away
> from where "nr" is actually used for updating those statistics: please
> move it down with them.  (I see "nr" is also reported in the tracepoint
> at the end, FWIW, so maybe that will show "0" in more failure cases than
> it used to, but that's okay - it has been decently initialized.)
>

Thanks Hugh! I will make sure V11 moves "nr" closer to the place it is used.

> Thanks,
> Hugh