On 29 Apr 2026, at 11:29, Zi Yan wrote: > collapse_file() requires FSes supporting large folio with at least > PMD_ORDER, so replace the READ_ONLY_THP_FOR_FS check with that. > MADV_COLLAPSE ignores shmem huge config, so exclude the check for shmem. > > While at it, replace VM_BUG_ON with VM_WARN_ON_ONCE. > > Add a helper function mapping_pmd_folio_support() for FSes supporting large > folio with at least PMD_ORDER. > > Signed-off-by: Zi Yan <[email protected]> > Reviewed-by: Lance Yang <[email protected]> > Reviewed-by: Baolin Wang <[email protected]> > --- > include/linux/pagemap.h | 26 ++++++++++++++++++++++++++ > mm/khugepaged.c | 10 ++++++++-- > 2 files changed, 34 insertions(+), 2 deletions(-) > > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h > index 1f50991b43e3b..1fed3414fe9b8 100644 > --- a/include/linux/pagemap.h > +++ b/include/linux/pagemap.h > @@ -513,6 +513,32 @@ static inline bool mapping_large_folio_support(const > struct address_space *mappi > return mapping_max_folio_order(mapping) > 0; > } > > +/** > + * mapping_pmd_folio_support() - Check if a mapping support PMD-sized folio > + * @mapping: The address_space > + * > + * Some file supports large folio but does not support as large as PMD order. > + * If a PMD-sized pagecache folio is attempted to be created on a filesystem, > + * this check needs to be performed first. > + * > + * Return: true - PMD-sized folio is supported, false - PMD-sized folio is > not > + * supported. > + */ > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE > +static inline bool mapping_pmd_folio_support(const struct address_space > *mapping) > +{ > + /* AS_FOLIO_ORDER is only reasonable for pagecache folios */ > + VM_WARN_ON_ONCE((unsigned long)mapping & FOLIO_MAPPING_ANON); > + > + return mapping_max_folio_order(mapping) >= PMD_ORDER; > +} > +#else > +static inline bool mapping_pmd_folio_support(const struct address_space > *mapping) > +{ > + return false; > +} > +#endif > + > /* Return the maximum folio size for this pagecache mapping, in bytes. */ > static inline size_t mapping_max_folio_size(const struct address_space > *mapping) > { > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index e112525c4aa9c..6808f2b48d864 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -2235,8 +2235,14 @@ static enum scan_result collapse_file(struct mm_struct > *mm, unsigned long addr, > int nr_none = 0; > bool is_shmem = shmem_file(file); > > - VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem); > - VM_BUG_ON(start & (HPAGE_PMD_NR - 1)); > + /* > + * MADV_COLLAPSE ignores shmem huge config, so do not check shmem > + * > + * TODO: once shmem always calls mapping_set_large_folios() on its > + * mapping, the shmem check can be removed. > + */ > + VM_WARN_ON_ONCE(!is_shmem && !mapping_pmd_folio_support(mapping));
sashiko asked: Is it possible for userspace to intentionally trigger this warning via madvise(MADV_COLLAPSE) on an unsupported read-only file? Although in a later commit, modified file_thp_enabled() will prevent this. Answer: It is possible when CONFIG_READ_ONLY_THP_FOR_FS is enabled, but it is unlikely a kernel will be shipped at this exact commit. > + VM_WARN_ON_ONCE(start & (HPAGE_PMD_NR - 1)); > > result = alloc_charge_folio(&new_folio, mm, cc, HPAGE_PMD_ORDER); > if (result != SCAN_SUCCEED) > -- > 2.53.0 Best Regards, Yan, Zi
