On 11/11/20 12:40 PM, Zi Yan wrote:
From: Zi Yan <ziy@xxxxxxxxxx>
To minimize the number of pages after a truncation, when truncating a
THP, we do not need to split it all the way down to order-0. The THP has
at most three parts, the part before offset, the part to be truncated,
the part left at the end. Use the non-zero minimum of them to decide
what order we split the THP to.
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
mm/truncate.c | 22 ++++++++++++++++++++--
1 file changed, 20 insertions(+), 2 deletions(-)
diff --git a/mm/truncate.c b/mm/truncate.c
index 20bd17538ec2..6d8e3c6115bc 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -237,7 +237,7 @@ int truncate_inode_page(struct address_space *mapping, struct page *page)
bool truncate_inode_partial_page(struct page *page, loff_t start, loff_t end)
{
loff_t pos = page_offset(page);
- unsigned int offset, length;
+ unsigned int offset, length, left, min_subpage_size = PAGE_SIZE;
Maybe use "remaining" instead of "left" since I think of the latter as the length of the
left side (offset).
if (pos < start)
offset = start - pos;
@@ -248,6 +248,7 @@ bool truncate_inode_partial_page(struct page *page, loff_t start, loff_t end)
length = length - offset;
else
length = end + 1 - pos - offset;
+ left = thp_size(page) - offset - length;
wait_on_page_writeback(page);
if (length == thp_size(page)) {
@@ -267,7 +268,24 @@ bool truncate_inode_partial_page(struct page *page, loff_t start, loff_t end)
do_invalidatepage(page, offset, length);
if (!PageTransHuge(page))
return true;
- return split_huge_page(page) == 0;
+
+ /*
+ * find the non-zero minimum of offset, length, and left and use it to
+ * decide the new order of the page after split
+ */
+ if (offset && left)
+ min_subpage_size = min_t(unsigned int,
+ min_t(unsigned int, offset, length),
+ left);
+ else if (!offset)
+ min_subpage_size = min_t(unsigned int, length, left);
+ else /* !left */
+ min_subpage_size = min_t(unsigned int, length, offset);
+
+ min_subpage_size = max_t(unsigned int, PAGE_SIZE, min_subpage_size);
+
+ return split_huge_page_to_list_to_order(page, NULL,
+ ilog2(min_subpage_size/PAGE_SIZE)) == 0;
}
What if "min_subpage_size" is 1/2 the THP but offset isn't aligned to 1/2?
Splitting the page in half wouldn't result in a page that could be freed
but maybe splitting to 1/4 would (assuming the THP is at least 8x PAGE_SIZE).
