Re: [PATCH v2 2/3] mm/memory-failure: improve large block size folio handling.

[Yang Shi <shy828301@gmail.com>]

On Mon, Oct 20, 2025 at 12:46 PM Zi Yan <ziy@nvidia.com> wrote:
>
> On 17 Oct 2025, at 15:11, Yang Shi wrote:
>
> > On Wed, Oct 15, 2025 at 8:38 PM Zi Yan <ziy@nvidia.com> wrote:
> >>
> >> Large block size (LBS) folios cannot be split to order-0 folios but
> >> min_order_for_folio(). Current split fails directly, but that is not
> >> optimal. Split the folio to min_order_for_folio(), so that, after split,
> >> only the folio containing the poisoned page becomes unusable instead.
> >>
> >> For soft offline, do not split the large folio if it cannot be split to
> >> order-0. Since the folio is still accessible from userspace and premature
> >> split might lead to potential performance loss.
> >>
> >> Suggested-by: Jane Chu <jane.chu@oracle.com>
> >> Signed-off-by: Zi Yan <ziy@nvidia.com>
> >> Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
> >> ---
> >>  mm/memory-failure.c | 25 +++++++++++++++++++++----
> >>  1 file changed, 21 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> >> index f698df156bf8..443df9581c24 100644
> >> --- a/mm/memory-failure.c
> >> +++ b/mm/memory-failure.c
> >> @@ -1656,12 +1656,13 @@ static int identify_page_state(unsigned long pfn, struct page *p,
> >>   * there is still more to do, hence the page refcount we took earlier
> >>   * is still needed.
> >>   */
> >> -static int try_to_split_thp_page(struct page *page, bool release)
> >> +static int try_to_split_thp_page(struct page *page, unsigned int new_order,
> >> +               bool release)
> >>  {
> >>         int ret;
> >>
> >>         lock_page(page);
> >> -       ret = split_huge_page(page);
> >> +       ret = split_huge_page_to_list_to_order(page, NULL, new_order);
> >>         unlock_page(page);
> >>
> >>         if (ret && release)
> >> @@ -2280,6 +2281,7 @@ int memory_failure(unsigned long pfn, int flags)
> >>         folio_unlock(folio);
> >>
> >>         if (folio_test_large(folio)) {
> >> +               int new_order = min_order_for_split(folio);
> >>                 /*
> >>                  * The flag must be set after the refcount is bumped
> >>                  * otherwise it may race with THP split.
> >> @@ -2294,7 +2296,14 @@ int memory_failure(unsigned long pfn, int flags)
> >>                  * page is a valid handlable page.
> >>                  */
> >>                 folio_set_has_hwpoisoned(folio);
> >> -               if (try_to_split_thp_page(p, false) < 0) {
> >> +               /*
> >> +                * If the folio cannot be split to order-0, kill the process,
> >> +                * but split the folio anyway to minimize the amount of unusable
> >> +                * pages.
> >> +                */
> >> +               if (try_to_split_thp_page(p, new_order, false) || new_order) {
> >
> > folio split will clear PG_has_hwpoisoned flag. It is ok for splitting
> > to order-0 folios because the PG_hwpoisoned flag is set on the
> > poisoned page. But if you split the folio to some smaller order large
> > folios, it seems you need to keep PG_has_hwpoisoned flag on the
> > poisoned folio.
>
> OK, this means all pages in a folio with folio_test_has_hwpoisoned() should be
> checked to be able to set after-split folio's flag properly. Current folio
> split code does not do that. I am thinking about whether that causes any
> issue. Probably not, because:
>
> 1. before Patch 1 is applied, large after-split folios are already causing
> a warning in memory_failure(). That kinda masks this issue.
> 2. after Patch 1 is applied, no large after-split folios will appear,
> since the split will fail.

I'm a little bit confused. Didn't this patch split large folio to
new-order-large-folio (new order is min order)? So this patch had
code:
if (try_to_split_thp_page(p, new_order, false) || new_order) {

Thanks,
Yang

>
> @Miaohe and @Jane, please let me know if my above reasoning makes sense or not.
>
> To make this patch right, folio's has_hwpoisoned flag needs to be preserved
> like what Yang described above. My current plan is to move
> folio_clear_has_hwpoisoned(folio) into __split_folio_to_order() and
> scan every page in the folio if the folio's has_hwpoisoned is set.
> There will be redundant scans in non uniform split case, since a has_hwpoisoned
> folio can be split multiple times (leading to multiple page scans), unless
> the scan result is stored.
>
> @Miaohe and @Jane, is it possible to have multiple HW poisoned pages in
> a folio? Is the memory failure process like 1) page access causing MCE,
> 2) memory_failure() is used to handle it and split the large folio containing
> it? Or multiple MCEs can be received and multiple pages in a folio are marked
> then a split would happen?
>
> >
> > Yang
> >
> >
> >> +                       /* get folio again in case the original one is split */
> >> +                       folio = page_folio(p);
> >>                         res = -EHWPOISON;
> >>                         kill_procs_now(p, pfn, flags, folio);
> >>                         put_page(p);
> >> @@ -2621,7 +2630,15 @@ static int soft_offline_in_use_page(struct page *page)
> >>         };
> >>
> >>         if (!huge && folio_test_large(folio)) {
> >> -               if (try_to_split_thp_page(page, true)) {
> >> +               int new_order = min_order_for_split(folio);
> >> +
> >> +               /*
> >> +                * If the folio cannot be split to order-0, do not split it at
> >> +                * all to retain the still accessible large folio.
> >> +                * NOTE: if getting free memory is perferred, split it like it
> >> +                * is done in memory_failure().
> >> +                */
> >> +               if (new_order || try_to_split_thp_page(page, new_order, true)) {
> >>                         pr_info("%#lx: thp split failed\n", pfn);
> >>                         return -EBUSY;
> >>                 }
> >> --
> >> 2.51.0
> >>
> >>
>
>
> --
> Best Regards,
> Yan, Zi