Re: [PATCH v6 2/6] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()
From: David Hildenbrand
Date: Mon Apr 08 2024 - 05:43:21 EST
+
+/**
+ * swap_pte_batch - detect a PTE batch for a set of contiguous swap entries
+ * @start_ptep: Page table pointer for the first entry.
+ * @max_nr: The maximum number of table entries to consider.
+ * @entry: Swap entry recovered from the first table entry.
+ *
+ * Detect a batch of contiguous swap entries: consecutive (non-present) PTEs
+ * containing swap entries all with consecutive offsets and targeting the same
+ * swap type.
+ *
Likely you should document that any swp pte bits are ignored? ()
Sorry I don't understand this comment. I thought any non-none, non-present PTE
was always considered to contain only a "swap entry" and a swap entry consists
of a "type" and an "offset" only. (and its a special "non-swap" swap entry if
type > SOME_CONSTANT) Are you saying there are additional fields in the PTE that
are not part of the swap entry?
pte_swp_soft_dirty()
pte_swp_clear_exclusive()
pte_swp_uffd_wp()
Are PTE bits used for swp PTE.
There is also dirty/young for migration entries, but that's not of a
concern here, because we stop for non_swap_entry().
+ * max_nr must be at least one and must be limited by the caller so scanning
+ * cannot exceed a single page table.
+ *
+ * Return: the number of table entries in the batch.
+ */
+static inline int swap_pte_batch(pte_t *start_ptep, int max_nr,
+ swp_entry_t entry)
+{
+ const pte_t *end_ptep = start_ptep + max_nr;
+ unsigned long expected_offset = swp_offset(entry) + 1;
+ unsigned int expected_type = swp_type(entry);
+ pte_t *ptep = start_ptep + 1;
+
+ VM_WARN_ON(max_nr < 1);
+ VM_WARN_ON(non_swap_entry(entry));
+
+ while (ptep < end_ptep) {
+ pte_t pte = ptep_get(ptep);
+
+ if (pte_none(pte) || pte_present(pte))
+ break;
+
+ entry = pte_to_swp_entry(pte);
+
+ if (non_swap_entry(entry) ||
+ swp_type(entry) != expected_type ||
+ swp_offset(entry) != expected_offset)
+ break;
+
+ expected_offset++;
+ ptep++;
+ }
+
+ return ptep - start_ptep;
+}
Looks very clean :)
I was wondering whether we could similarly construct the expected swp PTE and
only check pte_same.
expected_pte = __swp_entry_to_pte(__swp_entry(expected_type, expected_offset));
... or have a variant to increase only the swp offset for an existing pte. But
non-trivial due to the arch-dependent format.
But then, we'd fail on mismatch of other swp pte bits.
Hmm, perhaps I have a misunderstanding regarding "swp pte bits"...
On swapin, when reusing this function (likely!), we'll might to make sure that
the PTE bits match as well.
See below regarding uffd-wp.
#endif /* CONFIG_MMU */
void __acct_reclaim_writeback(pg_data_t *pgdat, struct folio *folio,
diff --git a/mm/madvise.c b/mm/madvise.c
index 1f77a51baaac..070bedb4996e 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -628,6 +628,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned
long addr,
struct folio *folio;
int nr_swap = 0;
unsigned long next;
+ int nr, max_nr;
next = pmd_addr_end(addr, end);
if (pmd_trans_huge(*pmd))
@@ -640,7 +641,8 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned
long addr,
return 0;
flush_tlb_batched_pending(mm);
arch_enter_lazy_mmu_mode();
- for (; addr != end; pte++, addr += PAGE_SIZE) {
+ for (; addr != end; pte += nr, addr += PAGE_SIZE * nr) {
+ nr = 1;
ptent = ptep_get(pte);
if (pte_none(ptent))
@@ -655,9 +657,11 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned
long addr,
entry = pte_to_swp_entry(ptent);
if (!non_swap_entry(entry)) {
- nr_swap--;
- free_swap_and_cache(entry);
- pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
+ max_nr = (end - addr) / PAGE_SIZE;
+ nr = swap_pte_batch(pte, max_nr, entry);
+ nr_swap -= nr;
+ free_swap_and_cache_nr(entry, nr);
+ clear_not_present_full_ptes(mm, addr, pte, nr, tlb->fullmm);
} else if (is_hwpoison_entry(entry) ||
is_poisoned_swp_entry(entry)) {
pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
diff --git a/mm/memory.c b/mm/memory.c
index 7dc6c3d9fa83..ef2968894718 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1637,12 +1637,13 @@ static unsigned long zap_pte_range(struct mmu_gather
*tlb,
folio_remove_rmap_pte(folio, page, vma);
folio_put(folio);
} else if (!non_swap_entry(entry)) {
- /* Genuine swap entry, hence a private anon page */
+ max_nr = (end - addr) / PAGE_SIZE;
+ nr = swap_pte_batch(pte, max_nr, entry);
+ /* Genuine swap entries, hence a private anon pages */
if (!should_zap_cows(details))
continue;
- rss[MM_SWAPENTS]--;
- if (unlikely(!free_swap_and_cache(entry)))
- print_bad_pte(vma, addr, ptent, NULL);
+ rss[MM_SWAPENTS] -= nr;
+ free_swap_and_cache_nr(entry, nr);
} else if (is_migration_entry(entry)) {
folio = pfn_swap_entry_folio(entry);
if (!should_zap_folio(details, folio))
@@ -1665,8 +1666,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
pr_alert("unrecognized swap entry 0x%lx\n", entry.val);
WARN_ON_ONCE(1);
}
- pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
- zap_install_uffd_wp_if_needed(vma, addr, pte, 1, details, ptent);
+ clear_not_present_full_ptes(mm, addr, pte, nr, tlb->fullmm);
For zap_install_uffd_wp_if_needed(), the uffd-wp bit has to match.
zap_install_uffd_wp_if_needed() will use the uffd-wp information in
ptent->pteval to make a decision whether to place PTE_MARKER_UFFD_WP markers.
On mixture, you either lose some or place too many markers.
What path are you concerned about here? I don't get how what you describe can
happen? swap_pte_batch() will only give me a batch of actual swap entries and
actual swap entries don't contain uffd-wp info, IIUC. If the function gets to a
"non-swap" swap entry, it bails. I thought the uffd-wp info was populated based
on the VMA state at swap-in? I think you are telling me that it's persisted
across the swap per-pte?
Please see zap_install_uffd_wp_if_needed():
if (unlikely(pte_swp_uffd_wp_any(pteval)))
arm_uffd_pte = true;
The PTEs (swp PTEs to be precise) contain uffd-wp informtation.
[...]
+ /*
+ * Short-circuit the below loop if none of the entries had their
+ * reference drop to zero.
+ */
+ if (!any_only_cache)
+ goto out;
- count = __swap_entry_free(p, entry);
- if (count == SWAP_HAS_CACHE)
- __try_to_reclaim_swap(p, swp_offset(entry),
+ /*
+ * Now go back over the range trying to reclaim the swap cache. This is
+ * more efficient for large folios because we will only try to reclaim
+ * the swap once per folio in the common case. If we do
+ * __swap_entry_free() and __try_to_reclaim_swap() in the same loop, the
+ * latter will get a reference and lock the folio for every individual
+ * page but will only succeed once the swap slot for every subpage is
+ * zero.
+ */
+ for (offset = swp_offset(entry); offset < end; offset += nr) {
+ nr = 1;
+ if (READ_ONCE(si->swap_map[offset]) == SWAP_HAS_CACHE) {
Here we use READ_ONCE() only, above data_race(). Hmmm.
Yes. I think this is correct.
READ_ONCE() is a "marked access" which KCSAN understands, so it won't complain
about it. So data_race() isn't required when READ_ONCE() (or WRITE_ONCE()) is
used. I believe READ_ONCE() is required here because we don't have a lock and we
want to make sure we read it in a non-tearing manner.
We don't need the READ_ONCE() above since we don't care about the exact value -
only that it's not 0 (because we should be holding a ref). So do a plain access
to give the compiler a bit more freedom. But we need to mark that with
data_race() to stop KCSAN from complaining.
Okay.
--
Cheers,
David / dhildenb