Re: [PATCH v4 2/3] mm: introduce put_user_page*(), placeholder versions

[Jan Kara]

On Tue 09-10-18 17:42:09, John Hubbard wrote:
> On 10/8/18 5:14 PM, Andrew Morton wrote:
> > Also, maintainability.  What happens if someone now uses put_page() by
> > mistake?  Kernel fails in some mysterious fashion?  How can we prevent
> > this from occurring as code evolves?  Is there a cheap way of detecting
> > this bug at runtime?
> > 
> 
> It might be possible to do a few run-time checks, such as "does page that came 
> back to put_user_page() have the correct flags?", but it's harder (without 
> having a dedicated page flag) to detect the other direction: "did someone page 
> in a get_user_pages page, to put_page?"
> 
> As Jan said in his reply, converting get_user_pages (and put_user_page) to 
> work with a new data type that wraps struct pages, would solve it, but that's
> an awfully large change. Still...given how much of a mess this can turn into 
> if it's wrong, I wonder if it's worth it--maybe? 

I'm certainly not opposed to looking into it. But after looking into this
for a while it is not clear to me how to convert e.g. fs/direct-io.c or
fs/iomap.c. They pass the reference from gup() via
bio->bi_io_vec[]->bv_page and then release it after IO completion.
Propagating the new type to ->bv_page is not good as lower layer do not
really care how the page is pinned in memory. But we do need to somehow
pass the information to the IO completion functions in a robust manner.

Hmm, what about the following:

1) Make gup() return new type - struct user_page *? In practice that would
be just a struct page pointer with 0 bit set so that people are forced to
use proper helpers and not just force types (and the setting of bit 0 and
masking back would be hidden behind CONFIG_DEBUG_USER_PAGE_REFERENCES for
performance reasons). Also the transition would have to be gradual so we'd
have to name the function differently and use it from converted code.

2) Provide helper bio_add_user_page() that will take user_page, convert it
to struct page, add it to the bio, and flag the bio as having pages with
user references. That code would also make sure the bio is consistent in
having only user-referenced pages in that case. IO completion (like
bio_check_pages_dirty(), bio_release_pages() etc.) will check the flag and
use approprite release function.

3) I have noticed fs/direct-io.c may submit zero page for IO when it needs
to clear stuff so we'll probably need a helper function to acquire 'user pin'
reference given a page pointer so that that code can be kept reasonably
simple and pass user_page references all around.

So this way we could maintain reasonable confidence that refcounts didn't
get mixed up. Thoughts?

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