Re: [PATCHv6 05/37] thp: try to free page's buffers before attempt split

Matthew Wilcox
2017-02-09T20:14:16Z

On Thu, Jan 26, 2017 at 02:57:47PM +0300, Kirill A. Shutemov wrote:
> @@ -2146,6 +2146,23 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> goto out;
> }
>
> + /* Try to free buffers before attempt split */
> + if (!PageSwapBacked(head) && PagePrivate(page)) {
> + /*
> + * We cannot trigger writeback from here due possible
> + * recursion if triggered from vmscan, only wait.
> + *
> + * Caller can trigger writeback it on its own, if safe.
> + */

It took me a few reads to get this. May I suggest:

/*
* Cannot split a page with buffers. If the caller has
* already started writeback, we can wait for it to finish,
* but we cannot start writeback if we were called from vmscan
*/
> + if (!PageSwapBacked(head) && PagePrivate(page)) {

Also, it looks weird to test PageSwapBacked of *head* and PagePrivate
of *page*. I think it's correct, but it still looks weird.

Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>