On Mon 24-04-23 20:48:32, Baolin Wang wrote: > > > On 4/24/2023 8:08 PM, Michal Hocko wrote: > > On Mon 24-04-23 19:40:30, Baolin Wang wrote: > > > > > > > > > On 4/24/2023 7:34 PM, Michal Hocko wrote: > > > > 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. > > > > > > Sure. > > > > > > > > > > > > > > > > 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? > > > > > > In the example I provided[2], these page frames of the hole memory are > > > zero-prefilled. > > > > OK, so make _that_ explicit in the comment. Essentially you want to say > > that there are cases where we have zero-initialized struct pages for > > memory holes. In general no pfn walker should touch a physical memory > > range for pfn where the struct page doesn't contain any metadata it > > recognizes. Zero fill struct pages do not contain any distinguishable > > state so that makes it less of a problem. > > > > All that being said I would reformulate the comment as follows: > > > > * Note: the function may return non-NULL struct page even for a > > * page block which contains a memory hole (i.e. there is no > > * physical memory for a subset of the pfn range). This should be > > * safe most of the time because struct pages are still zero > > * pre-filled and pfn walkers shouldn't touch any physical memory > > * range for which they do not recognize any specific metadata in > > * struct pages. > > Thanks. That makes sense to me. A trivial thing is I still want to add the > example in the comments to make it clear. Are you okay with below > description? > > + * Note: the function may return non-NULL struct page even for a page block > + * which contains a memory hole (i.e. there is no physical memory for a > subset > + * of the pfn range). 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 should be safe most > of > + * the time because struct pages are still zero pre-filled and pfn walkers > + * shouldn't touch any physical memory range for which they do not > recognize > + * any specific metadata in struct pages. No objections of course. I do not see an additional value, quite honestly but if somebody does then it doesn't hurt. -- Michal Hocko SUSE Labs
