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

[David Hildenbrand]

On 21.10.25 17:55, Zi Yan wrote:
> On 21 Oct 2025, at 11:44, David Hildenbrand wrote:
>
> > On 21.10.25 03:23, Zi Yan wrote:
> > > On 20 Oct 2025, at 19:41, Yang Shi wrote:
> > >
> > > > On Mon, Oct 20, 2025 at 12:46 PM Zi Yan <ziy@xxxxxxxxxx> wrote:
> > > > > On 17 Oct 2025, at 15:11, Yang Shi wrote:
> > > > >
> > > > > > On Wed, Oct 15, 2025 at 8:38 PM Zi Yan <ziy@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > > > > Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> > > > > > > Reviewed-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> > > > > > > ---
> > > > > > >    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) {
> > >
> > > Yes, but this is Patch 2 in this series. Patch 1 is
> > > "mm/huge_memory: do not change split_huge_page*() target order silently."
> > > and sent separately as a hotfix[1].
> >
> > I'm confused now as well. I'd like to review, will there be a v3 that only contains patch #2+#3?
>
> Yes. The new V3 will have 3 patches:
> 1. a new patch addresses Yang’s concern on setting has_hwpoisoned on after-split
> large folios.
> 2. patch#2,
> 3. patch#3.

Okay, I'll wait with the review until you resend :)

> The plan is to send them out once patch 1 is upstreamed. Let me know if you think
> it is OK to send them out earlier as Andrew already picked up patch 1.

It's in mm/mm-new + mm/mm-unstable, AFAIKT. So sure, send it against one 
of the tress (I prefer mm-unstable but usually we should target mm-new).

> I also would like to get some feedback on my approach to setting has_hwpoisoned:
>
> 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.

Oh, that's nasty indeed ... will have to think about that a bit.

Maybe we can keep it simple and always set folio_set_has_hwpoisoned() on 
all split folios? Essentially turning it into a "maybe_has" semantics.

IIUC, the existing folio_stest_has_hwpoisoned users can deal with that?

--
Cheers

David / dhildenb