Re: [RFC PATCH 0/7] block, fs: convert Direct IO to FOLL_PIN

[Jan Kara]

On Fri 25-02-22 00:50:18, John Hubbard wrote:
> Hi,
> 
> Summary:
> 
> This puts some prerequisites in place, including a CONFIG parameter,
> making it possible to start converting and testing the Direct IO part of
> each filesystem, from get_user_pages_fast(), to pin_user_pages_fast().
> 
> It will take "a few" kernel releases to get the whole thing done.
> 
> Details:
> 
> As part of fixing the "get_user_pages() + file-backed memory" problem
> [1], and to support various COW-related fixes as well [2], we need to
> convert the Direct IO code from get_user_pages_fast(), to
> pin_user_pages_fast(). Because pin_user_pages*() calls require a
> corresponding call to unpin_user_page(), the conversion is more
> elaborate than just substitution.
> 
> Further complicating the conversion, the block/bio layers get their
> Direct IO pages via iov_iter_get_pages() and iov_iter_get_pages_alloc(),
> each of which has a large number of callers. All of those callers need
> to be audited and changed so that they call unpin_user_page(), rather
> than put_page().
> 
> After quite some time exploring and consulting with people as well, it
> is clear that this cannot be done in just one patchset. That's because,
> not only is this large and time-consuming (for example, Chaitanya
> Kulkarni's first reaction, after looking into the details, was, "convert
> the remaining filesystems to use iomap, *then* convert to FOLL_PIN..."),
> but it is also spread across many filesystems.

With having modified fs/direct-io.c and fs/iomap/direct-io.c which
filesystems do you know are missing conversion? Or is it that you just want
to make sure with audit everything is fine? The only fs I could find
unconverted by your changes is ceph. Am I missing something?

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR