On Tue, Sep 22, 2020 at 04:39:06PM +0200, Christoph Hellwig wrote:

On Tue, Sep 22, 2020 at 12:21:44PM +0100, Matthew Wilcox wrote:

...

Actually, vfree() will work today; I cc'd you on a documentation update to make it clear that this is permitted.

vfree calls __free_pages, the i915 and a lot of other code calls put_page. They are mostly the same, but not quite and everytime I look into that mess I'm more confused than before.

Can someone in the know write sensible documentation on when to use __free_page(s) vs put_page?

I started on that, and then I found a bug that's been lurking for 12 years, so that delayed the documentation somewhat. The short answer is that __free_pages() lets you free non-compound high-order pages while put_page() can only free order-0 and compound pages.

I would really like to overhaul our memory allocation APIs:

current new __get_free_page(s) alloc_page(s) free_page(s) free_page(s) alloc_page(s) get_free_page(s) __free_pages put_page_order

Then put_page() and put_page_order() are more obviously friends.

But I cannot imagine a world in which Linus says yes to that upheaval. He's previous expressed dislike of the get_free_page() family of APIs, and thinks all those callers should just use kmalloc(). Maybe we can make that transition happen, now that kmalloc() aligns larger allocations.