Re: Fixing private mappings

[ebiederm+eric@npwt.net (Eric W. Biederman)]

>>>>> "ST" == Stephen C Tweedie <sct@dcs.ed.ac.uk> writes:

ST> Hi,
ST> On 23 Apr 1998 17:03:02 -0500, ebiederm+eric@npwt.net (Eric
ST> W. Biederman) said:

ST> No --- in the context of a MAP_PRIVATE mapping, only in-memory writes to
ST> the privately mapped virtual address space count as write references.  

Got it. 
I still like the semantics I defined, but if they aren't defined as
map_private I won't worry about it for the present.

Sometime it might be worth it/fun implementing a MAP_SNAPSHOT, but I
won't worry about that for the present.

ST> Yep, but we are not required to support non-page-aligned maps at all, so
ST> hacking it for special read-only cases is no big deal.  Doing a search
ST> for all overlapping mapped pages would be far too slow.

I think in the general case I could implement it without overhead and
in the common a.out case within a factor of 2, and in the worst case
within a factor of 4 (assuming a restriction of 1k alignment).  And
this is primarly memcpy cost there should be no need for extra disk
i/o.

The scheme I'm playing with using will share the same case as extra
huge file I/O (> 16TB), and in the common case should perform
identically to what we have now.

Thanks for setting me straight.  It hadn't been my intention to play
with mmap until I found this really weird use of that mmap makes of
the page_cache, so I really wasn't prepared for that one.

Eric