Re: [syzbot] [mm?] WARNING in memory_failure

[jane . chu]

On 10/1/2025 7:04 PM, Zi Yan wrote:
> On 1 Oct 2025, at 20:38, Zi Yan wrote:
>
> > On 1 Oct 2025, at 19:58, jane.chu@xxxxxxxxxx wrote:
> >
> > > Hi, Zi Yan,
> > >
> > > On 9/30/2025 9:51 PM, syzbot wrote:
> > > > Hello,
> > > >
> > > > syzbot has tested the proposed patch but the reproducer is still triggering an issue:
> > > > lost connection to test machine
> > > >
> > > >
> > > >
> > > > Tested on:
> > > >
> > > > commit:         d8795075 mm/huge_memory: do not change split_huge_page..
> > > > git tree:       https://github.com/x-y-z/linux-dev.git fix_split_page_min_order-for-kernelci
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=17ce96e2580000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=714d45b6135c308e
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=e6367ea2fdab6ed46056
> > > > compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> > > > userspace arch: arm64
> > > >
> > > > Note: no patches were applied.
> >
> > Thank you for looking into this.
> >
> > > My hunch is that
> > > https://github.com/x-y-z/linux-dev.git fix_split_page_min_order-for-kernelci
> > > alone is not enough.  Perhaps on ARM64, the page cache pages of /dev/nullb0 in
> > Yes, it only has the first patch, which fails a split if it cannot be
> > split to the intended order (order-0 in this case).
> >
> >
> > > the test case are probably with min_order > 0, therefore THP split fails, as the console message show:
> > > [  200.378989][T18221] Memory failure: 0x124d30: recovery action for unsplit thp: Failed
> > >
> > > With lots of poisoned THP pages stuck in the page cache, OOM could trigger too soon.
> >
> > That is my understanding too. Thanks for the confirmation.
> >
> > > I think it's worth to try add the additional changes I suggested earlier -
> > > https://lore.kernel.org/lkml/7577871f-06be-492d-b6d7-8404d7a045e0@xxxxxxxxxx/
> > >
> > > So that in the madvise HWPOISON cases, large huge pages are splitted to smaller huge pages, and most of them remain usable in the page cache.
> >
> > Yep, I am going to incorporate your suggestion as the second patch and make
> > syzbot check it again.
>
>
> #syz test: https://github.com/x-y-z/linux-dev.git fix_split_page_min_order_and_opt_memory_failure-for-kernelci

There is a bug here,

		if (try_to_split_thp_page(p, new_order, false) || new_order) {
			res = -EHWPOISON;
			kill_procs_now(p, pfn, flags, folio);  <---

If try_to_split_thp_page() succeeded on min_order, 'folio' should be 
retaken:  folio = page_folio(page) before moving on to kill_procs_now().

thanks,
-jane

> --
> Best Regards,
> Yan, Zi