[PATCH RFC 4/6] mm: support large folios swapin as a whole

Barry Song
2024-01-18T11:12:05Z

From: Chuanhua Han <hanchuanhua@oppo.com>

On an embedded system like Android, more than half of anon memory is actually
in swap devices such as zRAM. For example, while an app is switched to back-
ground, its most memory might be swapped-out.

Now we have mTHP features, unfortunately, if we don't support large folios
swap-in, once those large folios are swapped-out, we immediately lose the
performance gain we can get through large folios and hardware optimization
such as CONT-PTE.

This patch brings up mTHP swap-in support. Right now, we limit mTHP swap-in
to those contiguous swaps which were likely swapped out from mTHP as a whole.

On the other hand, the current implementation only covers the SWAP_SYCHRONOUS
case. It doesn't support swapin_readahead as large folios yet.

Right now, we are re-faulting large folios which are still in swapcache as a
whole, this can effectively decrease extra loops and early-exitings which we
have increased in arch_swap_restore() while supporting MTE restore for folios
rather than page.

Signed-off-by: Chuanhua Han <hanchuanhua@oppo.com>
Co-developed-by: Barry Song <v-songbaohua@oppo.com>
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
---
mm/memory.c | 108 +++++++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 94 insertions(+), 14 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index f61a48929ba7..928b3f542932 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -107,6 +107,8 @@ EXPORT_SYMBOL(mem_map);
static vm_fault_t do_fault(struct vm_fault *vmf);
static vm_fault_t do_anonymous_page(struct vm_fault *vmf);
static bool vmf_pte_changed(struct vm_fault *vmf);
+static struct folio *alloc_anon_folio(struct vm_fault *vmf,
+ bool (*pte_range_check)(pte_t *, int));

/*
* Return true if the original pte was a uffd-wp pte marker (so the pte was
@@ -3784,6 +3786,34 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
return VM_FAULT_SIGBUS;
}

+static bool pte_range_swap(pte_t *pte, int nr_pages)
+{
+ int i;
+ swp_entry_t entry;
+ unsigned type;
+ pgoff_t start_offset;
+
+ entry = pte_to_swp_entry(ptep_get_lockless(pte));
+ if (non_swap_entry(entry))
+ return false;
+ start_offset = swp_offset(entry);
+ if (start_offset % nr_pages)
+ return false;
+
+ type = swp_type(entry);
+ for (i = 1; i < nr_pages; i++) {
+ entry = pte_to_swp_entry(ptep_get_lockless(pte + i));
+ if (non_swap_entry(entry))
+ return false;
+ if (swp_offset(entry) != start_offset + i)
+ return false;
+ if (swp_type(entry) != type)
+ return false;
+ }
+
+ return true;
+}
+
/*
* We enter with non-exclusive mmap_lock (to exclude vma changes,
* but allow concurrent faults), and pte mapped but not yet locked.
@@ -3804,6 +3834,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
pte_t pte;
vm_fault_t ret = 0;
void *shadow = NULL;
+ int nr_pages = 1;
+ unsigned long start_address;
+ pte_t *start_pte;

if (!pte_unmap_same(vmf))
goto out;
@@ -3868,13 +3901,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
__swap_count(entry) == 1) {
/* skip swapcache */
- folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
- vma, vmf->address, false);
+ folio = alloc_anon_folio(vmf, pte_range_swap);
page = &folio->page;
if (folio) {
__folio_set_locked(folio);
__folio_set_swapbacked(folio);

+ if (folio_test_large(folio)) {
+ unsigned long start_offset;
+
+ nr_pages = folio_nr_pages(folio);
+ start_offset = swp_offset(entry) & ~(nr_pages - 1);
+ entry = swp_entry(swp_type(entry), start_offset);
+ }
+
if (mem_cgroup_swapin_charge_folio(folio,
vma->vm_mm, GFP_KERNEL,
entry)) {
@@ -3980,6 +4020,39 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
*/
vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
&vmf->ptl);
+
+ start_address = vmf->address;
+ start_pte = vmf->pte;
+ if (folio_test_large(folio)) {
+ unsigned long nr = folio_nr_pages(folio);
+ unsigned long addr = ALIGN_DOWN(vmf->address, nr * PAGE_SIZE);
+ pte_t *pte_t = vmf->pte - (vmf->address - addr) / PAGE_SIZE;
+
+ /*
+ * case 1: we are allocating large_folio, try to map it as a whole
+ * iff the swap entries are still entirely mapped;
+ * case 2: we hit a large folio in swapcache, and all swap entries
+ * are still entirely mapped, try to map a large folio as a whole.
+ * otherwise, map only the faulting page within the large folio
+ * which is swapcache
+ */
+ if (pte_range_swap(pte_t, nr)) {
+ start_address = addr;
+ start_pte = pte_t;
+ if (unlikely(folio == swapcache)) {
+ /*
+ * the below has been done before swap_read_folio()
+ * for case 1
+ */
+ nr_pages = nr;
+ entry = pte_to_swp_entry(ptep_get(start_pte));
+ page = &folio->page;
+ }
+ } else if (nr_pages > 1) { /* ptes have changed for case 1 */
+ goto out_nomap;
+ }
+ }
+
if (unlikely(!vmf->pte || !pte_same(ptep_get(vmf->pte), vmf->orig_pte)))
goto out_nomap;

@@ -4047,12 +4120,14 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
* We're already holding a reference on the page but haven't mapped it
* yet.
*/
- swap_free(entry);
+ swap_nr_free(entry, nr_pages);
if (should_try_to_free_swap(folio, vma, vmf->flags))
folio_free_swap(folio);

- inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
- dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
+ folio_ref_add(folio, nr_pages - 1);
+ add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
+ add_mm_counter(vma->vm_mm, MM_SWAPENTS, -nr_pages);
+
pte = mk_pte(page, vma->vm_page_prot);

/*
@@ -4062,14 +4137,14 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
* exclusivity.
*/
if (!folio_test_ksm(folio) &&
- (exclusive || folio_ref_count(folio) == 1)) {
+ (exclusive || folio_ref_count(folio) == nr_pages)) {
if (vmf->flags & FAULT_FLAG_WRITE) {
pte = maybe_mkwrite(pte_mkdirty(pte), vma);
vmf->flags &= ~FAULT_FLAG_WRITE;
}
rmap_flags |= RMAP_EXCLUSIVE;
}
- flush_icache_page(vma, page);
+ flush_icache_pages(vma, page, nr_pages);
if (pte_swp_soft_dirty(vmf->orig_pte))
pte = pte_mksoft_dirty(pte);
if (pte_swp_uffd_wp(vmf->orig_pte))
@@ -4081,14 +4156,15 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
folio_add_new_anon_rmap(folio, vma, vmf->address);
folio_add_lru_vma(folio, vma);
} else {
- folio_add_anon_rmap_pte(folio, page, vma, vmf->address,
+ folio_add_anon_rmap_ptes(folio, page, nr_pages, vma, start_address,
rmap_flags);
}

VM_BUG_ON(!folio_test_anon(folio) ||
(pte_write(pte) && !PageAnonExclusive(page)));
- set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
- arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
+ set_ptes(vma->vm_mm, start_address, start_pte, pte, nr_pages);
+
+ arch_do_swap_page(vma->vm_mm, vma, start_address, pte, vmf->orig_pte);

folio_unlock(folio);
if (folio != swapcache && swapcache) {
@@ -4105,6 +4181,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
}

if (vmf->flags & FAULT_FLAG_WRITE) {
+ if (folio_test_large(folio) && nr_pages > 1)
+ vmf->orig_pte = ptep_get(vmf->pte);
+
ret |= do_wp_page(vmf);
if (ret & VM_FAULT_ERROR)
ret &= VM_FAULT_ERROR;
@@ -4112,7 +4191,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
}

/* No need to invalidate - it was non-present before */
- update_mmu_cache_range(vmf, vma, vmf->address, vmf->pte, 1);
+ update_mmu_cache_range(vmf, vma, start_address, start_pte, nr_pages);
unlock:
if (vmf->pte)
pte_unmap_unlock(vmf->pte, vmf->ptl);
@@ -4148,7 +4227,8 @@ static bool pte_range_none(pte_t *pte, int nr_pages)
return true;
}

-static struct folio *alloc_anon_folio(struct vm_fault *vmf)
+static struct folio *alloc_anon_folio(struct vm_fault *vmf,
+ bool (*pte_range_check)(pte_t *, int))
{
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
struct vm_area_struct *vma = vmf->vma;
@@ -4190,7 +4270,7 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
order = highest_order(orders);
while (orders) {
addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
- if (pte_range_none(pte + pte_index(addr), 1 << order))
+ if (pte_range_check(pte + pte_index(addr), 1 << order))
break;
order = next_order(&orders, order);
}
@@ -4269,7 +4349,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
if (unlikely(anon_vma_prepare(vma)))
goto oom;
/* Returns NULL on OOM or ERR_PTR(-EAGAIN) if we must retry the fault */
- folio = alloc_anon_folio(vmf);
+ folio = alloc_anon_folio(vmf, pte_range_none);
if (IS_ERR(folio))
return 0;
if (!folio)
--
2.34.1