On 30.07.24 17:30, Marek Szyprowski wrote:
On 25.07.2024 20:39, David Hildenbrand wrote:
pte_lockptr() is the only *_lockptr() function that doesn't consume
what would be expected: it consumes a pmd_t pointer instead of a pte_t
pointer.
Let's change that. The two callers in pgtable-generic.c are easily
adjusted. Adjust khugepaged.c:retract_page_tables() to simply do a
pte_offset_map_nolock() to obtain the lock, even though we won't actually
be traversing the page table.
This makes the code more similar to the other variants and avoids other
hacks to make the new pte_lockptr() version happy. pte_lockptr() users
reside now only in pgtable-generic.c.
Maybe, using pte_offset_map_nolock() is the right thing to do because
the PTE table could have been removed in the meantime? At least it sounds
more future proof if we ever have other means of page table reclaim.
It's not quite clear if holding the PTE table lock is really required:
what if someone else obtains the lock just after we unlock it? But we'll
leave that as is for now, maybe there are good reasons.
This is a preparation for adapting hugetlb page table locking logic to
take the same locks as core-mm page table walkers would.
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
This patch landed in today's linux-next as commit e98970a1d2d4 ("mm: let
pte_lockptr() consume a pte_t pointer"). Unfortunately it causes the
following issue on most of my ARM 32bit based test boards:
That is ... rather surprising.
The issue below seems to point at __pte_offset_map_lock(), where we
essentially convert from
ptlock_ptr(page_ptdesc(pmd_page(*pmd)));
to
ptlock_ptr(virt_to_ptdesc(pte));
So we would get a NULL ptr from the ptdesc. Either the lock would
actually not be allocated or virt_to_ptdesc() does something unexpected.
Leaf page tables on arm are also a single page, so we cannot possibly be
staring at the wrong page-table-subpage.
Do we maybe have to special-case init-mm? But I don't see how that
special-casing would have happened for now :/
What's your kernel config value of SPLIT_PTLOCK_CPUS?
--
Cheers,
David / dhildenb
