Re: [PATCH 1/3] replace PAGECACHE_TAG_* definition with enumeration

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Mon, Jun 02, 2014 at 02:16:57PM -0700, Andrew Morton wrote:
> On Mon, 02 Jun 2014 09:12:25 -0700 Dave Hansen <dave.hansen@intel.com> wrote:
> 
> > On 06/01/2014 10:24 PM, Naoya Horiguchi wrote:
> > > -#define PAGECACHE_TAG_DIRTY	0
> > > -#define PAGECACHE_TAG_WRITEBACK	1
> > > -#define PAGECACHE_TAG_TOWRITE	2
> > > +enum {
> > > +	PAGECACHE_TAG_DIRTY,
> > > +	PAGECACHE_TAG_WRITEBACK,
> > > +	PAGECACHE_TAG_TOWRITE,
> > > +	__NR_PAGECACHE_TAGS,
> > > +};
> > 
> > Doesn't this end up exposing kernel-internal values out to a userspace
> > interface?  Wouldn't that lock these values in to the ABI?
> 
> Yes, we should be careful here.  We should not do anything which
> constrains future kernel code or which causes any form of
> compatibility/migration issues.

OK.

> I wonder if we can do something smart with the interface.  For example
> when userspace calls sys_fincore() it must explicitly ask for
> PAGECACHE_TAG_DIRTY and if some future kernel doesn't implement
> PAGECACHE_TAG_DIRTY, it can return -EINVAL.
> 
> Or maybe it can succeed, but tells userspace "you didn't get
> PAGECACHE_TAG_DIRTY".
> 
> <thinking out loud>
> 
> So userspace sends a mask of bits which select what fields it wants. 
> The kernel returns a mask of bits which tell userspace what it actually
> received.
> 
> Or something like that - you get the idea ;)

Thanks, I'll try this with another flag parameter for this purpose.

Naoya Horiguchi

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