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From: Andreas Gruenbacher <agruenba@redhat.com>
To: Steven Whitehouse <swhiteho@redhat.com>
Cc: cluster-devel <cluster-devel@redhat.com>, Jan Kara <jack@suse.cz>,
	LKML <linux-kernel@vger.kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Bob Peterson <rpeterso@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	ocfs2-devel@oss.oracle.com
Subject: Re: [Ocfs2-devel] [Cluster-devel] [PATCH v6 10/19] gfs2: Introduce flag for glock holder auto-demotion
Date: Mon, 23 Aug 2021 17:18:12 +0200	[thread overview]
Message-ID: <CAHc6FU5uHJSXD+CQk3W9BfZmnBCd+fqHt4Bd+=uVH18rnYCPLg@mail.gmail.com> (raw)
In-Reply-To: <8e2ab23b93c96248b7c253dc3ea2007f5244adee.camel@redhat.com>

On Mon, Aug 23, 2021 at 10:14 AM Steven Whitehouse <swhiteho@redhat.com> wrote:
> On Fri, 2021-08-20 at 17:22 +0200, Andreas Gruenbacher wrote:
> > On Fri, Aug 20, 2021 at 3:11 PM Bob Peterson <rpeterso@redhat.com>
> > wrote:
> > >
> [snip]
> > >
> > > You can almost think of this as a performance enhancement. This
> > > concept
> > > allows a process to hold a glock for much longer periods of time,
> > > at a
> > > lower priority, for example, when gfs2_file_read_iter needs to hold
> > > the
> > > glock for very long-running iterative reads.
> >
> > Consider a process that allocates a somewhat large buffer and reads
> > into it in chunks that are not page aligned. The buffer initially
> > won't be faulted in, so we fault in the first chunk and write into
> > it.
> > Then, when reading the second chunk, we find that the first page of
> > the second chunk is already present. We fill it, set the
> > HIF_MAY_DEMOTE flag, fault in more pages, and clear the
> > HIF_MAY_DEMOTE
> > flag. If we then still have the glock (which is very likely), we
> > resume the read. Otherwise, we return a short result.
> >
> > Thanks,
> > Andreas
> >
>
> If the goal here is just to allow the glock to be held for a longer
> period of time, but with occasional interruptions to prevent
> starvation, then we have a potential model for this. There is
> cond_resched_lock() which does this for spin locks.

This isn't an appropriate model for what I'm trying to achieve here.
In the cond_resched case, we know at the time of the cond_resched call
whether or not we want to schedule. If we do, we want to drop the spin
lock, schedule, and then re-acquire the spin lock. In the case we're
looking at here, we want to fault in user pages. There is no way of
knowing beforehand if the glock we're currently holding will have to
be dropped to achieve that. In fact, it will almost never have to be
dropped. But if it does, we need to drop it straight away to allow the
conflicting locking request to succeed.

Have a look at how the patch queue uses gfs2_holder_allow_demote() and
gfs2_holder_disallow_demote():

https://listman.redhat.com/archives/cluster-devel/2021-August/msg00128.html
https://listman.redhat.com/archives/cluster-devel/2021-August/msg00134.html

Thanks,
Andreas


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  reply	other threads:[~2021-08-23 15:18 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-19 19:40 [Ocfs2-devel] [PATCH v6 00/19] gfs2: Fix mmap + page fault deadlocks Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 01/19] iov_iter: Fix iov_iter_get_pages{, _alloc} page fault return value Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 02/19] powerpc/kvm: Fix kvm_use_magic_page Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 03/19] Turn fault_in_pages_{readable, writeable} into fault_in_{readable, writeable} Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 04/19] Turn iov_iter_fault_in_readable into fault_in_iov_iter_readable Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 05/19] iov_iter: Introduce fault_in_iov_iter_writeable Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 06/19] gfs2: Add wrapper for iomap_file_buffered_write Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 07/19] gfs2: Clean up function may_grant Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 08/19] gfs2: Eliminate vestigial HIF_FIRST Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 09/19] gfs2: Remove redundant check from gfs2_glock_dq Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 10/19] gfs2: Introduce flag for glock holder auto-demotion Andreas Gruenbacher
2021-08-20  9:35   ` [Ocfs2-devel] [Cluster-devel] " Steven Whitehouse
2021-08-20 13:11     ` Bob Peterson
2021-08-20 13:41       ` Steven Whitehouse
2021-08-20 15:22       ` Andreas Gruenbacher
2021-08-23  8:14         ` Steven Whitehouse
2021-08-23 15:18           ` Andreas Gruenbacher [this message]
2021-08-23 16:05             ` Matthew Wilcox
2021-08-23 16:36               ` Bob Peterson
2021-08-23 19:12               ` Andreas Gruenbacher
2021-08-24  7:59             ` Steven Whitehouse
2021-08-20 13:17     ` Andreas Gruenbacher
2021-08-20 13:47       ` Steven Whitehouse
2021-08-20 14:43         ` Andreas Grünbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 11/19] gfs2: Move the inode glock locking to gfs2_file_buffered_write Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 12/19] gfs2: Fix mmap + page fault deadlocks for buffered I/O Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 13/19] iomap: Fix iomap_dio_rw return value for user copies Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 14/19] iomap: Support partial direct I/O on user copy failures Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 15/19] iomap: Add done_before argument to iomap_dio_rw Andreas Gruenbacher
2021-08-19 19:40 ` [Ocfs2-devel] [PATCH v6 16/19] gup: Introduce FOLL_NOFAULT flag to disable page faults Andreas Gruenbacher
2021-08-19 19:41 ` [Ocfs2-devel] [PATCH v6 17/19] iov_iter: Introduce nofault " Andreas Gruenbacher
2021-08-19 19:41 ` [Ocfs2-devel] [PATCH v6 18/19] gfs2: Fix mmap + page fault deadlocks for direct I/O Andreas Gruenbacher
2021-08-19 19:41 ` [Ocfs2-devel] [PATCH v6 19/19] gfs2: Eliminate ip->i_gh Andreas Gruenbacher

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