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

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Message-ID: <20200625204044.GH7703@casper.infradead.org>
Date:   Thu, 25 Jun 2020 21:40:44 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        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, Jun 25, 2020 at 10:36:11PM +0200, Michal Hocko wrote:
> 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.

If XFS needs to track transactions, why doesn't it use
current->journal_info like btrfs/ceph/ext4/gfs2/nilfs2/reiserfs?