On 20.06.23 01:10, Peter Xu wrote:
The acceleration of THP was done with ctx.page_mask, however it'll be
ignored if **pages is non-NULL.
The old optimization was introduced in 2013 in 240aadeedc4a ("mm:
accelerate mm_populate() treatment of THP pages"). It didn't explain why
we can't optimize the **pages non-NULL case. It's possible that at that
time the major goal was for mm_populate() which should be enough back then.
In the past we had these sub-page refcounts for THP. My best guess (and
I didn't check if that was still the case in 2013) would be that it was
simpler regarding refcount handling to to do it one-subpage at a time.
But I might be just wrong.
Optimize thp for all cases, by properly looping over each subpage, doing
cache flushes, and boost refcounts / pincounts where needed in one go.
This can be verified using gup_test below:
# chrt -f 1 ./gup_test -m 512 -t -L -n 1024 -r 10
Before: 13992.50 ( +-8.75%)
After: 378.50 (+-69.62%)
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
mm/gup.c | 51 ++++++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 44 insertions(+), 7 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c
index 4a00d609033e..b50272012e49 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1199,16 +1199,53 @@ static long __get_user_pages(struct mm_struct *mm,
goto out;
}
next_page:
- if (pages) {
- pages[i] = page;
- flush_anon_page(vma, page, start);
- flush_dcache_page(page);
- ctx.page_mask = 0;
- }
-
page_increm = 1 + (~(start >> PAGE_SHIFT) & ctx.page_mask);
if (page_increm > nr_pages)
page_increm = nr_pages;
+
+ if (pages) {
+ struct page *subpage;
+ unsigned int j;
+
+ /*
+ * This must be a large folio (and doesn't need to
+ * be the whole folio; it can be part of it), do
+ * the refcount work for all the subpages too.
+ *
+ * NOTE: here the page may not be the head page
+ * e.g. when start addr is not thp-size aligned.
+ * try_grab_folio() should have taken care of tail
+ * pages.
+ */
+ if (page_increm > 1) {
+ struct folio *folio;
+
+ /*
+ * Since we already hold refcount on the
+ * large folio, this should never fail.
+ */
+ folio = try_grab_folio(page, page_increm - 1,
+ foll_flags);
+ if (WARN_ON_ONCE(!folio)) {
+ /*
+ * Release the 1st page ref if the
+ * folio is problematic, fail hard.
+ */
+ gup_put_folio(page_folio(page), 1,
+ foll_flags);
+ ret = -EFAULT;
+ goto out;
+ }
+ }
+
+ for (j = 0; j < page_increm; j++) {
+ subpage = nth_page(page, j);
+ pages[i+j] = subpage;
Doe checkpatch like pages[i+j]? I'd have used spaces around the +.
+ flush_anon_page(vma, subpage, start + j * PAGE_SIZE);
+ flush_dcache_page(subpage);
+ }
+ }
+
i += page_increm;
start += page_increm * PAGE_SIZE;
nr_pages -= page_increm;
So, we did the first try_grab_folio() while our page was PMD-mapped
udner the PT lock and we had sufficient permissions (e.g., mapped
writable, no unsharing required). With FOLL_PIN, we incremented the
pincount.
I was wondering if something could have happened ever since we unlocked
the PT table lock and possibly PTE-mapped the THP. ... but as it's
already pinned, it cannot get shared during fork() [will stay exclusive].
So we can just take additional pins on that folio.
LGTM, although I do like the GUP-fast way of recording+ref'ing it at a
central place (see gup_huge_pmd() with record_subpages() and friends),
not after the effects.
--
Cheers,
David / dhildenb
