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From: Steven Whitehouse <swhiteho@redhat.com>
To: Andreas Gruenbacher <agruenba@redhat.com>,
	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>,
	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 09:14:50 +0100	[thread overview]
Message-ID: <8e2ab23b93c96248b7c253dc3ea2007f5244adee.camel@redhat.com> (raw)
In-Reply-To: <CAHc6FU7EMOEU7C5ryu5pMMx1v+8CTAOMyGdf=wfaw8=TTA_btQ@mail.gmail.com>

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. So perhaps we might
do something similar:

/**
 * gfs2_glock_cond_regain - Conditionally drop and regain glock
 * @gl: The glock
 * @gh: A granted holder for the glock
 *
 * If there is a pending demote request for this glock, drop and 
 * requeue a lock request for this glock. If there is no pending
 * demote request, this is a no-op. In either case the glock is
 * held on both entry and exit.
 *
 * Returns: 0 if no pending demote, 1 if lock dropped and regained
 */
int gfs2_glock_cond_regain(struct gfs2_glock *gl, struct gfs2_holder
*gh);

That seems more easily understood, and clearly documents places where
the lock may be dropped and regained. I think that the implementation
should be simpler and cleaner, compared with the current proposed
patch. There are only two bit flags related to pending demotes, for
example, so the check should be trivial.

It may need a few changes depending on the exact circumstances, but
hopefully that illustrates the concept,

Steve.




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  reply	other threads:[~2021-08-23  8:15 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 [this message]
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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