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Date:   Thu, 15 Aug 2019 07:56:30 +1000
From:   Dave Chinner <david@fromorbit.com>
To:     Jeff Layton <jlayton@kernel.org>
Cc:     ira.weiny@intel.com, Andrew Morton <akpm@linux-foundation.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Dan Williams <dan.j.williams@intel.com>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        Theodore Ts'o <tytso@mit.edu>,
        John Hubbard <jhubbard@nvidia.com>,
        Michal Hocko <mhocko@suse.com>, linux-xfs@vger.kernel.org,
        linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-nvdimm@lists.01.org,
        linux-ext4@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC PATCH v2 02/19] fs/locks: Add Exclusive flag to user Layout
 lease
Message-ID: <20190814215630.GQ6129@dread.disaster.area>
References: <20190809225833.6657-1-ira.weiny@intel.com>
 <20190809225833.6657-3-ira.weiny@intel.com>
 <fde2959db776616008fc5d31df700f5d7d899433.camel@kernel.org>
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On Wed, Aug 14, 2019 at 10:15:06AM -0400, Jeff Layton wrote:
> On Fri, 2019-08-09 at 15:58 -0700, ira.weiny@intel.com wrote:
> > From: Ira Weiny <ira.weiny@intel.com>
> > 
> > Add an exclusive lease flag which indicates that the layout mechanism
> > can not be broken.
> > 
> > Exclusive layout leases allow the file system to know that pages may be
> > GUP pined and that attempts to change the layout, ie truncate, should be
> > failed.
> > 
> > A process which attempts to break it's own exclusive lease gets an
> > EDEADLOCK return to help determine that this is likely a programming bug
> > vs someone else holding a resource.
.....
> > diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
> > index baddd54f3031..88b175ceccbc 100644
> > --- a/include/uapi/asm-generic/fcntl.h
> > +++ b/include/uapi/asm-generic/fcntl.h
> > @@ -176,6 +176,8 @@ struct f_owner_ex {
> >  
> >  #define F_LAYOUT	16      /* layout lease to allow longterm pins such as
> >  				   RDMA */
> > +#define F_EXCLUSIVE	32      /* layout lease is exclusive */
> > +				/* FIXME or shoudl this be F_EXLCK??? */
> >  
> >  /* operations for bsd flock(), also used by the kernel implementation */
> >  #define LOCK_SH		1	/* shared lock */
> 
> This interface just seems weird to me. The existing F_*LCK values aren't
> really set up to be flags, but are enumerated values (even if there are
> some gaps on some arches). For instance, on parisc and sparc:

I don't think we need to worry about this - the F_WRLCK version of
the layout lease should have these exclusive access semantics (i.e
other ops fail rather than block waiting for lease recall) and hence
the API shouldn't need a new flag to specify them.

i.e. the primary difference between F_RDLCK and F_WRLCK layout
leases is that the F_RDLCK is a shared, co-operative lease model
where only delays in operations will be seen, while F_WRLCK is a
"guarantee exclusive access and I don't care what it breaks"
model... :)

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com