Re: [PATCH 0/6] Overhaul memalloc_no*

Michal Hocko
2020-06-25T20:36:17Z

On Thu 25-06-20 11:48:32, Darrick J. Wong wrote:
> On Thu, Jun 25, 2020 at 12:31:16PM +0100, Matthew Wilcox (Oracle) wrote:
> > I want a memalloc_nowait like we have memalloc_noio and memalloc_nofs
> > for an upcoming patch series, and Jens also wants it for non-blocking
> > io_uring. It turns out we already have dm-bufio which could benefit
> > from memalloc_nowait, so it may as well go into the tree now.
> >
> > The biggest problem is that we're basically out of PF_ flags, so we need
> > to find somewhere else to store the PF_MEMALLOC_NOWAIT flag. It turns
> > out the PF_ flags are really supposed to be used for flags which are
> > accessed from other tasks, and the MEMALLOC flags are only going to
> > be used by this task. So shuffling everything around frees up some PF
> > flags and generally makes the world a better place.
>
> So, uh, how does this intersect with the patch "xfs: reintroduce
> PF_FSTRANS for transaction reservation recursion protection" that
> re-adds PF_TRANS because uh I guess we lost some subtlety or another at
> some point?

This is independent, really. It just relocates the NOFS flag. PF_TRANS
is reintroduced for a different reason. When I have replaced the
original PF_TRANS by PF_MEMALLOC_NOFS I didn't realized that xfs doesn't
need only the NOFS semantic but also the transaction tracking so this
cannot be a single bit only. So it has to be added back. But
PF_MEMALLOC_NOFS needs to stay for the scoped NOFS semantic.

Hope this clarifies it a bit.
--
Michal Hocko
SUSE Labs