Re: [RFC PATCH 11/13] mm/vas: Introduce VAS segments - shareable address space regions

From: Till Smejkal <till.smejkal@googlemail.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Till Smejkal <till.smejkal@googlemail.com>,
	Richard Henderson <rth@twiddle.net>,
	Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
	Matt Turner <mattst88@gmail.com>,
	Vineet Gupta <vgupta@synopsys.com>,
	Russell King <linux@armlinux.org.uk>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Steven Miao <realmz6@gmail.com>,
	Richard Kuo <rkuo@codeaurora.org>,
	Tony Luck <tony.luck@intel.com>,
	Fenghua Yu <fenghua.yu@intel.com>,
	James Hogan <james.hogan@imgtec.com>,
	Ralf Baechle <ralf@linux-mips.org>,
	"James E.J. Bottomley" <jejb@parisc-linux.org>,
	Helge Deller <deller@gmx.de>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Yoshinori Sato <ysato@users.sourceforge.jp>, Rich Felker <dalia>
Subject: Re: [RFC PATCH 11/13] mm/vas: Introduce VAS segments - shareable address space regions
Date: Mon, 13 Mar 2017 15:45:36 -0700	[thread overview]
Message-ID: <20170313224536.saqdijtdayxwmlpz@arch-dev> (raw)
In-Reply-To: <20170313222758.GB4033@bombadil.infradead.org>

Hi Matthew,

On Mon, 13 Mar 2017, Matthew Wilcox wrote:
> On Mon, Mar 13, 2017 at 03:14:13PM -0700, Till Smejkal wrote:
> > +/**
> > + * Create a new VAS segment.
> > + *
> > + * @param[in] name:		The name of the new VAS segment.
> > + * @param[in] start:		The address where the VAS segment begins.
> > + * @param[in] end:		The address where the VAS segment ends.
> > + * @param[in] mode:		The access rights for the VAS segment.
> > + *
> > + * @returns:			The VAS segment ID on success, -ERRNO otherwise.
> > + **/
> 
> Please follow the kernel-doc conventions, as described in
> Documentation/doc-guide/kernel-doc.rst.  Also, function documentation
> goes with the implementation, not the declaration.

Thank you for this pointer. I wasn't aware of this convention. I will change the
patches accordingly.

> > +/**
> > + * Get ID of the VAS segment belonging to a given name.
> > + *
> > + * @param[in] name:		The name of the VAS segment for which the ID
> > + *				should be returned.
> > + *
> > + * @returns:			The VAS segment ID on success, -ERRNO
> > + *				otherwise.
> > + **/
> > +extern int vas_seg_find(const char *name);
> 
> So ... segments have names, and IDs ... and access permissions ...
> Why isn't this a special purpose filesystem?

We also thought about this. However, we decided against implementing them as a
special purpose filesystem, mainly because we could not think of a good way to
represent a VAS/VAS segment in this file system (should they be represented rather as
file or directory) and we weren't sure what a hierarchy in the filesystem would mean
for the underlying address spaces. Hence we decided against it and rather used a
combination of IDR and sysfs. However, I don't have any strong feelings and would
also reimplement them as a special purpose filesystem if people rather like them to
be one.

Till

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