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

[Matthew Wilcox <willy@infradead.org>]

On Wed, May 10, 2023 at 09:35:44PM -0700, Hugh Dickins wrote:
> On Wed, 10 May 2023, Matthew Wilcox wrote:
> > On Tue, May 09, 2023 at 09:39:13PM -0700, Hugh Dickins wrote:
> > > Two: pte_offset_map() will need to do an rcu_read_lock(), with the
> > > corresponding rcu_read_unlock() in pte_unmap().  But most architectures
> > > never supported CONFIG_HIGHPTE, so some don't always call pte_unmap()
> > > after pte_offset_map(), or have used userspace pte_offset_map() where
> > > pte_offset_kernel() is more correct.  No problem in the current tree,
> > > but a problem once an rcu_read_unlock() will be needed to keep balance.
> > 
> > Hi Hugh,
> > 
> > I shall have to spend some time looking at these patches, but at LSFMM
> > just a few hours ago, I proposed and nobody objected to removing
> > CONFIG_HIGHPTE.  I don't intend to take action on that consensus
> > immediately, so I can certainly wait until your patches are applied, but
> > if this information simplifies what you're doing, feel free to act on it.
> 
> Thanks a lot, Matthew: very considerate, as usual.
> 
> Yes, I did see your "Whither Highmem?" (wither highmem!) proposal on the

I'm glad somebody noticed the pun ;-)

> list, and it did make me think, better get these patches and preview out
> soon, before you get to vanish pte_unmap() altogether.  HIGHMEM or not,
> HIGHPTE or not, I think pte_offset_map() and pte_unmap() still have an
> important role to play.
> 
> I don't really understand why you're going down a remove-CONFIG_HIGHPTE
> route: I thought you were motivated by the awkardness of kmap on large
> folios; but I don't see how removing HIGHPTE helps with that at all
> (unless you have a "large page tables" effort in mind, but I doubt it).

Quite right, my primary concern is filesystem metadata; primarily
directories as I don't think anybody has ever supported symlinks or
superblocks larger than 4kB.

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.

> But I've no investment in CONFIG_HIGHPTE if people think now is the
> time to remove it: I disagree, but wouldn't miss it myself - so long
> as you leave pte_offset_map() and pte_unmap() (under whatever names).
> 
> I don't think removing CONFIG_HIGHPTE will simplify what I'm doing.
> For a moment it looked like it would: the PAE case is nasty (and our
> data centres have not been on PAE for a long time, so it wasn't a
> problem I had to face before); and knowing pmd_high must be 0 for a
> page table looked like it would help, but now I'm not so sure of that
> (hmm, I'm changing my mind again as I write).
> 
> Peter's pmdp_get_lockless() does rely for complete correctness on
> interrupts being disabled, and I suspect that I may be forced in the
> PAE case to do so briefly; but detest that notion.  For now I'm just
> deferring it, hoping for a better idea before third series finalized.
> 
> I mention this (and Cc Peter) in passing: don't want this arch thread
> to go down into that rabbit hole: we can start a fresh thread on it if
> you wish, but right now my priority is commit messages for the second
> series, rather than solving (or even detailing) the PAE problem.

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?

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.