Re: O->A loan

[yamt%mwd.biglobe.ne.jp@localhost (YAMAMOTO Takashi)]

hi,

i made read with O->A loaning work for easy cases (ie. no locking difficulty)
on yamt-pagecache branch so that someone interested can benchmark.

YAMAMOTO Takashi

> hi,
> 
>> On Tue, Nov 29, 2011 at 06:38:27AM +0000, YAMAMOTO Takashi wrote:
>>> O->A loaned pages installed on the user address space would have a different
>>> owner than the usual map->entry.uvm_obj.
>>> although it was not a problem when you wrote this patch, at least some
>>> non-mechanical changes would be required after the recent locking
>>> changes in this area.  namely, uvm_map_lock_entry etc now assumes that
>>> any pages mapped in a map entry belong to either the entry's amap or
>>> underlying object.
>> 
>> ok, I didn't think it would be entirely mechanical.  :-)
>> 
>> what if the O->A loan code also changed the entry's uvm_obj to be the vnode
>> that the pages really belong to?  if the loan range in the amap is fully
>> populated (which it is in this context) then that shouldn't affect the
>> logical contents of the entry, it would just cause anyone locking the entry
>> to also lock the vnode.  if the range of the loan is smaller than the
>> range of the entry, we could split the entry.  do you think that would work?
> 
> it might work, but i have some concerns:
> - entry fragmentation
> - the extra uobj reference keeps the file even after unlink
> 
> YAMAMOTO Takashi
> 
>> 
>> -Chuck