Re: [PATCH 00/23] arch: allow pte_offset_map[_lock]() to fail

From: Hugh Dickins
Date: Thu May 11 2023 - 18:37:30 EST

On Thu, 11 May 2023, Matthew Wilcox wrote:
> I was thinking that removing CONFIG_HIGHPTE might simplify the page
> fault handling path a little, but now I've looked at it some more, and
> I'm not sure there's any simplification to be had. It should probably
> use kmap_local instead of kmap_atomic(), though.

Re kmap_local, yes, one of the patches in the next series does make
that change.

> I infer that what you need is a pte_access_start() and a
> pte_access_end() which look like they can be plausibly rcu_read_lock()
> and rcu_read_unlock(), but might need to be local_irq_save() and
> local_irq_restore() in some configurations?

Yes, except that the local_irq_restore() in PAE-like configurations
(if we need it at all) is not delayed until the pte_access_end() or
pte_unmap() - it's internal to the pte_access_start() or pte_offset_map():
interrupts only disabled across the getting of a consistent pmd entry.

Over-generalizing a little, any user of pte_offset_map() (as opposed to
pte_offset_map_lock()) has to be prepared for the ptes to change under
them: but we do need to give them something that is or was recently the
relevant page table, rather than a random page mishmashed from mismatched
pmd_low and pmd_high.

> We also talked about moving x86 to always RCU-free page tables in
> order to make accessing /proc/$pid/smaps lockless. I believe Michel
> is going to take a swing at this project.

(And /proc/$pid/numa_maps, I hope: that's even worse in some way, IIRC.)

That might be orthogonal to what I'm doing: many non-x86 architectures
already do RCU-freeing of page tables via the TLB route, but that doesn't
cover a pte_free() from retract_page_tables() or collapse_and_free_pmd().