On 12 Nov 2020, at 17:08, Ralph Campbell wrote:
On 11/11/20 12:40 PM, Zi Yan wrote:
From: Zi Yan ziy@nvidia.com
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@nvidia.com
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).
Sure. Will change the name.
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).
Is it possible? The whole THP is divided into three parts, offset, length, and remaining (renamed from left). If offset is not aligned to 1/2, it is either greater than 1/2 or smaller than 1/2. If it is the former, length and remaining will be smaller than 1/2, so min_subpage_size cannot be 1/2. If it is the latter, min_subpage_size cannot be 1/2 either. Because min_subpage_size is the smallest non-zero value of offset, length, and remaining. Let me know if I miss anything.
— Best Regards, Yan Zi