Re: [PATCH v2] infiniband/mm: convert put_page() to put_user_page*()

Matthew Wilcox
2019-05-26T11:06:37Z

On Fri, May 24, 2019 at 06:45:22PM -0700, john.hubbard@gmail.com wrote:
> For infiniband code that retains pages via get_user_pages*(),
> release those pages via the new put_user_page(), or
> put_user_pages*(), instead of put_page()

I have no objection to this particular patch, but ...

> This is a tiny part of the second step of fixing the problem described
> in [1]. The steps are:
>
> 1) Provide put_user_page*() routines, intended to be used
> for releasing pages that were pinned via get_user_pages*().
>
> 2) Convert all of the call sites for get_user_pages*(), to
> invoke put_user_page*(), instead of put_page(). This involves dozens of
> call sites, and will take some time.
>
> 3) After (2) is complete, use get_user_pages*() and put_user_page*() to
> implement tracking of these pages. This tracking will be separate from
> the existing struct page refcounting.
>
> 4) Use the tracking and identification of these pages, to implement
> special handling (especially in writeback paths) when the pages are
> backed by a filesystem. Again, [1] provides details as to why that is
> desirable.

I thought we agreed at LSFMM that the future is a new get_user_bvec()
/ put_user_bvec(). This is largely going to touch the same places as
step 2 in your list above. Is it worth doing step 2?

One of the advantages of put_user_bvec() is that it would be quite easy
to miss a conversion from put_page() to put_user_page(), but it'll be
a type error to miss a conversion from put_page() to put_user_bvec().