Re: [External] Re: [PATCH v2] mm/madvise: avoid skipping pages after splitting large folios

Hi Andrew,

On Thu, Aug 6, 2026 at 2:29 PM Andrew Morton <[email protected]> wrote:
>
> On Thu,  6 Aug 2026 13:55:01 +0800 Yunhui Cui <[email protected]> wrote:
>
> > madvise_inject_error() advances through the requested range using the
> > size of the page returned by get_user_pages_fast(). Saving the size
> > before error injection is required for hugetlb pages because successful
> > soft offlining can dissolve the source huge page.
> >
> > That stride is incorrect for non-hugetlb large folios in system memory.
> > The memory failure handlers split such a folio and handle only the base
> > page for the supplied PFN. Advancing by the pre-split folio size then
> > skips the remaining pages in the requested range while madvise() still
> > reports success.
> >
> > Advance by PAGE_SIZE for non-hugetlb folios in system memory. Retain
> > folio_size() for hugetlb and ZONE_DEVICE folios, as compound Device DAX
> > folios are handled as a whole.
>
> Thanks.
>
> > Fixes: 19bfbe22f59a ("mm, hugetlb, soft_offline: save compound page order before page migration")
> > Cc: [email protected]
> > Signed-off-by: Yunhui Cui <[email protected]>
>
> Should there be an Assisted-by: here?
>
> When fixing a bug, please always describe the userspace-visible runtime
> effects of that bug.  Especially when proposing a -stable backport.
>
>
> I asked Gemini this question and then pasted in your email.  It told me
> stuff, but I don't know if it's correct.  In this case it would be good
> to hear that description in your own words, please.
>
> And perhaps a means of reproducing the bug - clearly our selftests/
> coverage here is poor.

The issue can be reproduced with the softoffline helper from
avocado-misc-tests. With 64K mTHP enabled:
./softoffline -m private -n 50
the test consistently failed with:
pfn matches, softoffline failed at 10

After temporarily disabling 64K mTHP:
echo never > /sys/kernel/mm/transparent_hugepage/hugepages-64kB/enabled
./softoffline -m private -n 50
the test passed in five consecutive runs:
Softoffline succeeded!

Thanks,
Yunhui