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From: Andreas Gruenbacher <agruenba@redhat.com>
To: Bob Peterson <rpeterso@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>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Steven Whitehouse <swhiteho@redhat.com>,
	ocfs2-devel@oss.oracle.com
Subject: Re: [Ocfs2-devel] [Cluster-devel] [PATCH v6 10/19] gfs2: Introduce flag for glock holder auto-demotion
Date: Fri, 20 Aug 2021 17:22:54 +0200	[thread overview]
Message-ID: <CAHc6FU7EMOEU7C5ryu5pMMx1v+8CTAOMyGdf=wfaw8=TTA_btQ@mail.gmail.com> (raw)
In-Reply-To: <cf284633-a9db-9f88-6b60-4377bc33e473@redhat.com>

On Fri, Aug 20, 2021 at 3:11 PM Bob Peterson <rpeterso@redhat.com> wrote:
> On 8/20/21 4:35 AM, Steven Whitehouse wrote:
> > Hi,
> >
> > On Thu, 2021-08-19 at 21:40 +0200, Andreas Gruenbacher wrote:
> >> From: Bob Peterson <rpeterso@redhat.com>
> >>
> >> This patch introduces a new HIF_MAY_DEMOTE flag and infrastructure
> >> that
> >> will allow glocks to be demoted automatically on locking conflicts.
> >> When a locking request comes in that isn't compatible with the
> >> locking
> >> state of a holder and that holder has the HIF_MAY_DEMOTE flag set,
> >> the
> >> holder will be demoted automatically before the incoming locking
> >> request
> >> is granted.
> >>
> > I'm not sure I understand what is going on here. When there are locking
> > conflicts we generate call backs and those result in glock demotion.
> > There is no need for a flag to indicate that I think, since it is the
> > default behaviour anyway. Or perhaps the explanation is just a bit
> > confusing...
>
> I agree that the whole concept and explanation are confusing. Andreas
> and I went through several heated arguments about the semantics,
> comments, patch descriptions, etc. We played around with many different
> flag name ideas, etc. We did not agree on the best way to describe the
> whole concept. He didn't like my explanation and I didn't like his. So
> yes, it is confusing.
>
> My preferred terminology was "DOD" or "Dequeue On Demand"

... which is useless because it adds no clarity as to whose demand
we're talking about.

> which makes
> the concept more understandable to me. So basically a process can say
> "I need to hold this glock, but for an unknown and possibly lengthy
> period of time, but please feel free to dequeue it if it's in your way."
> And bear in mind that several processes may do the same, simultaneously.
>
> 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


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  parent reply	other threads:[~2021-08-20 15:24 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 [this message]
2021-08-23  8:14         ` Steven Whitehouse
2021-08-23 15:18           ` Andreas Gruenbacher
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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