On Wed, 31 May 2023, Jann Horn wrote:

On Mon, May 29, 2023 at 8:15 AM Hugh Dickins [off-list ref] wrote:

quoted

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.

[...]

quoted

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);