Re: [RFC] Changing COW detection to be memory hotplug friendly

[Andrea Arcangeli <andrea@suse.de>]

On Thu, Feb 10, 2005 at 08:19:47PM +0000, Hugh Dickins wrote:
> On Thu, 10 Feb 2005, Andrea Arcangeli wrote:
> > 
> > The reason pinned pages cannot be unmapped is that if they're being
> > unmapped while a rawio read is in progress, a cow on some shared
> > swapcache under I/O could happen.
> > 
> > If a try_to_unmap + COW over a shared swapcache happens during a rawio
> > read, then the rawio reads will get lost generating data corruption.
> 
> Yes, but...
> 
> get_user_pages broke COW in advance of the I/O.  The reason for
> subsequent COW if the page gets unmapped is precisely because
> can_share_swap_page used page_count to judge exclusivity, and
> get_user_pages has bumped that up, so the page appears (in danger
> of being) non-exclusive when actually it is exclusive.  By changing
> can_share_swap_page to use page_mapcount, that frustration vanishes.

What if a second thread does a fork() while the rawio is in progress?
The page will be again no shareable, and if you unmap it the rawio will
be currupt if the thread that executed the fork while the I/O is in
progress writes to a part of the page again after it has been unmapped
(obviously the part of the page that isn't under read-rawio, rawio works
with the hardblocksize, not on the whole page).