Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes: > Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(), which > checks whether the given zone contains holes, and uses pfn_to_online_page() > to validate if the start pfn is online and valid, as well as using pfn_valid() > to validate the end pfn. > > However, the __pageblock_pfn_to_page() function may return non-NULL even > if the end pfn of a pageblock is in a memory hole in some situations. For > example, if the pageblock order is MAX_ORDER, which will fall into 2 > sub-sections, and the end pfn of the pageblock may be hole even though > the start pfn is online and valid. > > This did not break anything until now, but the zone continuous is fragile > in this possible scenario. So as previous discussion[1], it is better to > add some comments to explain this possible issue in case there are some > future pfn walkers that rely on this. > > [1] https://lore.kernel.org/all/87r0sdsmr6.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> Looks good to me! Thanks! Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx> > --- > Changes from v1: > - Update the comments per Ying and Mike, thanks. > --- > mm/page_alloc.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 6457b64fe562..9756d66f471c 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -1502,6 +1502,13 @@ void __free_pages_core(struct page *page, unsigned int order) > * interleaving within a single pageblock. It is therefore sufficient to check > * the first and last page of a pageblock and avoid checking each individual > * page in a pageblock. > + * > + * Note: the function may return non-NULL even if the end pfn of a pageblock > + * is in a memory hole in some situations. For example, if the pageblock > + * order is MAX_ORDER, which will fall into 2 sub-sections, and the end pfn > + * of the pageblock may be hole even though the start pfn is online and valid. > + * This did not break anything until now, but be careful about this possible > + * issue when checking whether all pfns of a pageblock are valid. > */ > struct page *__pageblock_pfn_to_page(unsigned long start_pfn, > unsigned long end_pfn, struct zone *zone)
