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, though the start pfn of a pageblock is valid, it can not always > guarantee the end pfn of the pageblock is also valid (may be holes) in some > cases. For example, if the pageblock order is MAX_ORDER - 1, 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> > --- > mm/page_alloc.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 6457b64fe562..dc4005b32ae0 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -1502,6 +1502,14 @@ 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: if the start pfn of a pageblock is valid, but it can not always guarantee > + * the end pfn of the pageblock is also valid (may be holes) in some cases. For "valid" sounds confusing here. pfn_valid() is true, but the pfn is considered invalid at some degree. How about the following? 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 - 1, 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 this possible issue when checking if the whole pfns are valid of a ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ whether all pfns of a pageblock are valid. ? > + * pageblock. > */ > struct page *__pageblock_pfn_to_page(unsigned long start_pfn, > unsigned long end_pfn, struct zone *zone) My English is poor. So, feel free to ignore the comments. Best Regards, Huang, Ying
