Re: [PATCH v2 1/1] mm/mlock: implement folio_mlock_step() using folio_pte_batch()
From: David Hildenbrand
Date: Mon Jun 03 2024 - 11:27:19 EST
On 03.06.24 17:01, Matthew Wilcox wrote:
On Mon, Jun 03, 2024 at 04:56:05PM +0200, David Hildenbrand wrote:
On 03.06.24 16:43, Matthew Wilcox wrote:
On Mon, Jun 03, 2024 at 10:07:45PM +0800, Lance Yang wrote:
+++ b/mm/mlock.c
@@ -307,26 +307,15 @@ void munlock_folio(struct folio *folio)
static inline unsigned int folio_mlock_step(struct folio *folio,
pte_t *pte, unsigned long addr, unsigned long end)
{
- unsigned int count, i, nr = folio_nr_pages(folio);
- unsigned long pfn = folio_pfn(folio);
+ const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
+ unsigned int count = (end - addr) >> PAGE_SHIFT;
This is a pre-existing bug, but ... what happens if you're on a 64-bit
system and you mlock() a range that is exactly 2^44 bytes? Seems to me
that count becomes 0. Why not use an unsigned long here and avoid the
problem entirely?
folio_pte_batch() also needs to take an unsigned long max_nr in that
case, because you aren't restricting it to folio_nr_pages().
Yeah, likely we should also take a look at other folio_pte_batch() users
like copy_present_ptes() that pass the count as an int. Nothing should
really be broken, but we might not batch as much as we could, which is
unfortunate.
You did include:
VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio);
so at the least we have a userspace-triggerable warning.
Yes, and max_nr == 0 would likely not be healthy to the system.
But
For copy_pte_range(), zap_pte_range() and the madvise users, we should
always have:
next = pmd_addr_end(addr, end);
and use "next" as the actual "end" -- not the VMA end. So "end - addr" =
"next - addr" should never exceed a single PMD size.
mlock_pte_range() is also called from walk_page_range(), which uses
next = pmd_addr_end(addr, end);
So likely exceeding PMD size is not possible here and all is working as
expected.
Will double check later.
--
Cheers,
David / dhildenb