Re: [PATCH v2 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page()

[Michal Hocko]

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