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

[Hugh Dickins]

On Wed, 31 May 2023, Jason Gunthorpe wrote:
> 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.

No, I'm making no judgment of other places where pmdp_get_lockless() is
used: examination might show that some need more care, but I'll just
assume that each is taking as much care as it needs.

But here where I'm making changes, I do see that we need this extra care.

> 
> 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@xxxxxxxxxxxxxx/

Thanks a lot for the link: I don't know why, but I never saw that mail
thread at all before.  I have not fully digested it yet, to be honest:
MADV_DONTNEED, doesn't flush TLB yet, etc - I'll have to get into the
right frame of mind for that.

> 
> Though we never fixed it, AFAIK..

I'm certainly depending very much on pmdp_get_lockless(): and hoping to
find its case is easier to defend than at the ptep_get_lockless() level.

Thanks,
Hugh