linux-kernel - Re: [PATCH 0/6] Overhaul memalloc

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Message-ID: <20200625203611.GS1320@dhcp22.suse.cz>
Date:   Thu, 25 Jun 2020 22:36:11 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-xfs@...r.kernel.org, dm-devel@...hat.com,
        Mikulas Patocka <mpatocka@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, NeilBrown <neilb@...e.de>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH 0/6] Overhaul memalloc_no*

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