more strict KPI for vm_pager_get_pages()

[Gleb Smirnoff]

On Mon, May 04, 2015 at 05:50:39PM +0800, Julian Elischer wrote:
J> > IMO, the response from vm_pager_haspages() is only advisory, since
J> > filesystem might not control the external entities which are the source
J> > of the required data.
J> 
J> Also since the backing object is not locked, a truncate() may be performed
J> between the operations making the prior return information invalid.
J> Certainly in remote filesystems, possibly on local ones too.

Of course, the object shouldn't be unlocked between vm_pager_haspage()
and vm_pager_get_pages(). All current consumers do not unlock.

-- 
Totus tuus, Glebius.