Re: [PATCH 10/10] mm/migrate.c: call detach_page_private to cleanup code

[Matthew Wilcox]

On Tue, May 19, 2020 at 06:06:19PM +0800, Gao Xiang wrote:
> In addition, I found some limitation of new {attach,detach}_page_private
> helper (that is why I was interested in this series at that time [1] [2],
> but I gave up finally) since many patterns (not all) in EROFS are
> 
> io_submit (origin, page locked):
> attach_page_private(page);
> ...
> put_page(page);
> 
> end_io (page locked):
> SetPageUptodate(page);
> unlock_page(page);
> 
> since the page is always locked, so io_submit could be simplified as
> set_page_private(page, ...);
> SetPagePrivate(page);
> , which can save both one temporary get_page(page) and one
> put_page(page) since it could be regarded as safe with page locked.

It's fine to use page private like this without incrementing the refcount,
and I can't find any problematic cases in EROFS like those fixed by commit
8e47a457321ca1a74ad194ab5dcbca764bc70731

So I think the new helpers are not for you, and that's fine.  They'll be
useful for other filesystems which are using page_private differently
from the way that you do.