Re: [PATCH 00/34] put_user_pages(): miscellaneous call sites

Jan Kara
2019-08-09T08:34:35Z

On Wed 07-08-19 19:36:37, Ira Weiny wrote:
> On Wed, Aug 07, 2019 at 10:46:49AM +0200, Michal Hocko wrote:
> > > So I think your debug option and my suggested renaming serve a bit
> > > different purposes (and thus both make sense). If you do the renaming, you
> > > can just grep to see unconverted sites. Also when someone merges new GUP
> > > user (unaware of the new rules) while you switch GUP to use pins instead of
> > > ordinary references, you'll get compilation error in case of renaming
> > > instead of hard to debug refcount leak without the renaming. And such
> > > conflict is almost bound to happen given the size of GUP patch set... Also
> > > the renaming serves against the "coding inertia" - i.e., GUP is around for
> > > ages so people just use it without checking any documentation or comments.
> > > After switching how GUP works, what used to be correct isn't anymore so
> > > renaming the function serves as a warning that something has really
> > > changed.
> >
> > Fully agreed!
>
> Ok Prior to this I've been basing all my work for the RDMA/FS DAX stuff in
> Johns put_user_pages()... (Including when I proposed failing truncate with a
> lease in June [1])
>
> However, based on the suggestions in that thread it became clear that a new
> interface was going to need to be added to pass in the "RDMA file" information
> to GUP to associate file pins with the correct processes...
>
> I have many drawings on my white board with "a whole lot of lines" on them to
> make sure that if a process opens a file, mmaps it, pins it with RDMA, _closes_
> it, and ummaps it; that the resulting file pin can still be traced back to the
> RDMA context and all the processes which may have access to it.... No matter
> where the original context may have come from. I believe I have accomplished
> that.
>
> Before I go on, I would like to say that the "imbalance" of get_user_pages()
> and put_page() bothers me from a purist standpoint... However, since this
> discussion cropped up I went ahead and ported my work to Linus' current master
> (5.3-rc3+) and in doing so I only had to steal a bit of Johns code... Sorry
> John... :-(
>
> I don't have the commit messages all cleaned up and I know there may be some
> discussion on these new interfaces but I wanted to throw this series out there
> because I think it may be what Jan and Michal are driving at (or at least in
> that direction.
>
> Right now only RDMA and DAX FS's are supported. Other users of GUP will still
> fail on a DAX file and regular files will still be at risk.[2]
>
> I've pushed this work (based 5.3-rc3+ (33920f1ec5bf)) here[3]:
>
> https://github.com/weiny2/linux-kernel/tree/linus-rdmafsdax-b0-v3
>
> I think the most relevant patch to this conversation is:
>
> https://github.com/weiny2/linux-kernel/commit/5d377653ba5cf11c3b716f904b057bee6641aaf6
>
> I stole Jans suggestion for a name as the name I used while prototyping was
> pretty bad... So Thanks Jan... ;-)

For your function, I'd choose a name like vaddr_pin_leased_pages() so that
association with a lease is clear from the name :) Also I'd choose the
counterpart to be vaddr_unpin_leased_page[s](). Especially having put_page in
the name looks confusing to me...

Honza

--
Jan Kara <jack@suse.com>
SUSE Labs, CR