Re: [PATCH 01/12] mm/pgtable: add rcu_read_lock() and rcu_read_unlock()s

Hugh Dickins
2023-06-02T02:50:24Z

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On Wed, 31 May 2023, Jann Horn wrote:
> On Mon, May 29, 2023 at 8:15 AM Hugh Dickins <hughd@google.com> wrote:
> > Before putting them to use (several commits later), add rcu_read_lock()
> > to pte_offset_map(), and rcu_read_unlock() to pte_unmap(). Make this a
> > separate commit, since it risks exposing imbalances: prior commits have
> > fixed all the known imbalances, but we may find some have been missed.
> [...]
> > diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> > index c7ab18a5fb77..674671835631 100644
> > --- a/mm/pgtable-generic.c
> > +++ b/mm/pgtable-generic.c
> > @@ -236,7 +236,7 @@ pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
> > {
> > pmd_t pmdval;
> >
> > - /* rcu_read_lock() to be added later */
> > + rcu_read_lock();
> > pmdval = pmdp_get_lockless(pmd);
> > if (pmdvalp)
> > *pmdvalp = pmdval;
>
> It might be a good idea to document that this series assumes that the
> first argument to __pte_offset_map() is a pointer into a second-level
> page table (and not a local copy of the entry) unless the containing
> VMA is known to not be THP-eligible or the page table is detached from
> the page table hierarchy or something like that. Currently a bunch of
> places pass references to local copies of the entry, and while I think
> all of these are fine, it would probably be good to at least document
> why these are allowed to do it while other places aren't.

Thanks Jann: but I have to guess that here you are showing awareness of
an important issue that I'm simply ignorant of.

I have been haunted by a dim recollection that there is one architecture
(arm-32?) which is fussy about the placement of the pmdval being examined
(deduces info missing from the arch-independent interface, by following
up the address?), but I couldn't track it down when I tried.

Please tell me more; or better, don't spend your time explaining to me,
but please just send a link to a good reference on the issue. I'll be
unable to document what you ask there, without educating myself first.

Thanks,
Hugh

>
> $ vgrep 'pte_offset_map(&'
> Index File Line Content
> 0 arch/sparc/mm/tlb.c 151 pte = pte_offset_map(&pmd, vaddr);
> 1 kernel/events/core.c 7501 ptep = pte_offset_map(&pmd, addr);
> 2 mm/gup.c 2460 ptem = ptep = pte_offset_map(&pmd, addr);
> 3 mm/huge_memory.c 2057 pte = pte_offset_map(&_pmd, haddr);
> 4 mm/huge_memory.c 2214 pte = pte_offset_map(&_pmd, haddr);
> 5 mm/page_table_check.c 240 pte_t *ptep = pte_offset_map(&pmd, addr);