[PATCH 02/12] mm/pgtable: add PAE safety to __pte_offset_map()

[Jason Gunthorpe]

On Sun, May 28, 2023 at 11:16:16PM -0700, Hugh Dickins wrote:
> There is a faint risk that __pte_offset_map(), on a 32-bit architecture
> with a 64-bit pmd_t e.g. x86-32 with CONFIG_X86_PAE=y, would succeed on
> a pmdval assembled from a pmd_low and a pmd_high which never belonged
> together: their combination not pointing to a page table at all, perhaps
> not even a valid pfn.  pmdp_get_lockless() is not enough to prevent that.
> 
> Guard against that (on such configs) by local_irq_save() blocking TLB
> flush between present updates, as linux/pgtable.h suggests.  It's only
> needed around the pmdp_get_lockless() in __pte_offset_map(): a race when
> __pte_offset_map_lock() repeats the pmdp_get_lockless() after getting the
> lock, would just send it back to __pte_offset_map() again.

What about the other places calling pmdp_get_lockless ? It seems like
this is quietly making it part of the API that the caller must hold
the IPIs off.

And Jann had a note that this approach used by the lockless functions
doesn't work anyhow:

https://lore.kernel.org/linux-mm/CAG48ez3h-mnp9ZFC10v+-BW_8NQvxbwBsMYJFP8JX31o0B17Pg@mail.gmail.com/

Though we never fixed it, AFAIK..

Jason