On Mon 24-04-23 19:20:43, Baolin Wang wrote: > > > On 4/24/2023 5:54 PM, Michal Hocko wrote: > > On Sun 23-04-23 18:59:11, Baolin Wang wrote: > > > 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/ > > > > Do I remember correctly you've had a specific configuration that would > > trigger this case? > > Yes, I provided an example in previous thread [2] so show the > __pageblock_pfn_to_page() is fragile in some cases. > > [2] https://lore.kernel.org/all/52dfdd2e-9c99-eac4-233e-59919a24323e@xxxxxxxxxxxxxxxxx/ Please make it a part of the changelog. > > > > > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> > > > --- > > > 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. > > > > It is not really clear what you should be doing (other than to be > > careful which is not helpful much TBH) when you encounter this > > situation. If the reality changes and this would break in the future > > what would breakage look like? What should be done about that? > > That depends on what the future pfn walkers do, which may access some hole > memory with zero-init page frame. For example, if checking the > __PageMovable() for a zero-init page frame, that will crash the system. But > I can not list all the possible cases. > > So how about below words? > > * 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, that may > * lead to accessing empty page frame, and the worst case can crash the > system. > * So you should use pfn_to_onlie_page() instead of pfn_valid() to valid the > * pfns in a pageblock if such case happens. Does that mean that struct page is not initialized and PagePoisoned will trigger or it is just zero-prefilled? -- Michal Hocko SUSE Labs
