Re: [PATCH v3 6/6] mm: swap: entirely map large folios found in swapcache
From: David Hildenbrand
Date: Tue May 07 2024 - 04:24:42 EST
On 07.05.24 00:58, Barry Song wrote:
On Tue, May 7, 2024 at 1:16 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
On 06.05.24 14:58, Barry Song wrote:
On Tue, May 7, 2024 at 12:38 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
On Tue, May 7, 2024 at 12:07 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
On 04.05.24 01:23, Barry Song wrote:
On Fri, May 3, 2024 at 6:50 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
On 03/05/2024 01:50, Barry Song wrote:
From: Chuanhua Han <hanchuanhua@xxxxxxxx>
When a large folio is found in the swapcache, the current implementation
requires calling do_swap_page() nr_pages times, resulting in nr_pages
page faults. This patch opts to map the entire large folio at once to
minimize page faults. Additionally, redundant checks and early exits
for ARM64 MTE restoring are removed.
Signed-off-by: Chuanhua Han <hanchuanhua@xxxxxxxx>
Co-developed-by: Barry Song <v-songbaohua@xxxxxxxx>
Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
With the suggested changes below:
Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
---
mm/memory.c | 60 ++++++++++++++++++++++++++++++++++++++++++-----------
1 file changed, 48 insertions(+), 12 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c
index 22e7c33cc747..940fdbe69fa1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3968,6 +3968,10 @@ 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 page_idx = 0;
+ unsigned long address = vmf->address;
+ pte_t *ptep;
nit: Personally I'd prefer all these to get initialised just before the "if
(folio_test_large()..." block below. That way it is clear they are fresh (incase
any logic between here and there makes an adjustment) and its clear that they
are only to be used after that block (the compiler will warn if using an
uninitialized value).
right. I agree this will make the code more readable.
if (!pte_unmap_same(vmf))
goto out;
@@ -4166,6 +4170,36 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
goto out_nomap;
}
+ ptep = vmf->pte;
+ if (folio_test_large(folio) && folio_test_swapcache(folio)) {
+ int nr = folio_nr_pages(folio);
+ unsigned long idx = folio_page_idx(folio, page);
+ unsigned long folio_start = vmf->address - idx * PAGE_SIZE;
+ unsigned long folio_end = folio_start + nr * PAGE_SIZE;
+ pte_t *folio_ptep;
+ pte_t folio_pte;
+
+ if (unlikely(folio_start < max(vmf->address & PMD_MASK, vma->vm_start)))
+ goto check_folio;
+ if (unlikely(folio_end > pmd_addr_end(vmf->address, vma->vm_end)))
+ goto check_folio;
+
+ folio_ptep = vmf->pte - idx;
+ folio_pte = ptep_get(folio_ptep);
+ if (!pte_same(folio_pte, pte_move_swp_offset(vmf->orig_pte, -idx)) ||
+ swap_pte_batch(folio_ptep, nr, folio_pte) != nr)
+ goto check_folio;
+
+ page_idx = idx;
+ address = folio_start;
+ ptep = folio_ptep;
+ nr_pages = nr;
+ entry = folio->swap;
+ page = &folio->page;
+ }
+
+check_folio:
Is this still the correct label name, given the checks are now above the new
block? Perhaps "one_page" or something like that?
not quite sure about this, as the code after one_page can be multiple_pages.
On the other hand, it seems we are really checking folio after "check_folio"
:-)
BUG_ON(!folio_test_anon(folio) && folio_test_mappedtodisk(folio));
BUG_ON(folio_test_anon(folio) && PageAnonExclusive(page));
/*
* Check under PT lock (to protect against concurrent fork() sharing
* the swap entry concurrently) for certainly exclusive pages.
*/
if (!folio_test_ksm(folio)) {
+
/*
* PG_anon_exclusive reuses PG_mappedtodisk for anon pages. A swap pte
* must never point at an anonymous page in the swapcache that is
@@ -4225,12 +4259,13 @@ 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_nr(entry, 1);
+ swap_free_nr(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);
/*
@@ -4240,34 +4275,35 @@ 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 &&
+ folio_nr_pages(folio) == nr_pages))) {
I think in practice there is no change here? If nr_pages > 1 then the folio is
in the swapcache, so there is an extra ref on it? I agree with the change for
robustness sake. Just checking my understanding.
This is the code showing we are reusing/(mkwrite) a folio either
1. we meet a small folio and we are the only one hitting the small folio
2. we meet a large folio and we are the only one hitting the large folio
any corner cases besides the above two seems difficult. for example,
while we hit a large folio in swapcache but if we can't entirely map it
(nr_pages==1) due to partial unmap, we will have folio_ref_count(folio)
== nr_pages == 1
No, there would be other references from the swapcache and
folio_ref_count(folio) > 1. See my other reply.
right. can be clearer by:
Wait, do we still need folio_nr_pages(folio) == nr_pages even if we use
folio_ref_count(folio) == 1 and moving folio_ref_add(folio, nr_pages - 1)?
I don't think that we will "need" it.
one case is that we have a large folio with 16 PTEs, and we unmap
15 swap PTE entries, thus we have only one swap entry left. Then
we hit the large folio in swapcache. but we have only one PTE thus we will
map only one PTE. lacking folio_nr_pages(folio) == nr_pages, we reuse the
large folio for one PTE. with it, do_wp_page() will migrate the large
folio to a small one?
We will set PAE bit and do_wp_page() will unconditionally reuse that page.
Note that this is the same as if we had pte_swp_exclusive() set and
would have run into "exclusive=true" here.
If we'd want a similar "optimization" as we have in
wp_can_reuse_anon_folio(), you'd want something like
exclusive || (folio_ref_count(folio) == 1 &&
(!folio_test_large(folio) || nr_pages > 1)
I feel like
A : !folio_test_large(folio) || nr_pages > 1
equals
B: folio_nr_pages(folio) == nr_pages
if folio is small, folio_test_large(folio) is false, both A and B will be true;
if folio is large, and we map the whole large folio, A will be true
because of nr_pages > 1;
B is also true;
if folio is large, and we map single one PTE, A will be false;
B is also false, because nr_pages == 1 but folio_nr_pages(folio) > 1;
right?
Let's assume a single subpage of a large folio is no longer mapped.
Then, we'd have:
nr_pages == folio_nr_pages(folio) - 1.
You could simply map+reuse most of the folio without COWing.
Once we support COW reuse of PTE-mapped THP we'd do the same. Here, it's
just easy to detect that the folio is exclusive (folio_ref_count(folio)
== 1 before mapping anything).
If you really want to mimic what do_wp_page() currently does, you should
have:
exclusive || (folio_ref_count(folio) == 1 && !folio_test_large(folio))
Personally, I think we should keep it simple here and use:
exclusive || folio_ref_count(folio) == 1
IMHO, that's as clear as it gets.
--
Cheers,
David / dhildenb