Re: [PATCH] KVM swapping with MMU Notifiers V7

[Andrea Arcangeli <andrea@qumranet.com>]

On Sat, Feb 16, 2008 at 05:51:38AM -0600, Robin Holt wrote:
> I am doing this in xpmem with a stack-based structure in the function
> calling get_user_pages.  That structure describes the start and
> end address of the range we are doing the get_user_pages on.  If an
> invalidate_range_begin comes in while we are off to the kernel doing
> the get_user_pages, the invalidate_range_begin marks that structure
> indicating an invalidate came in.  When the get_user_pages gets the
> structures relocked, it checks that flag (really a generation counter)
> and if it is set, retries the get_user_pages.  After 3 retries, it
> returns -EAGAIN and the fault is started over from the remote side.

A seqlock sounds a good optimization for the non-swapping fast path, a
per-VM-guest seqlock number can allow us to know when we need to worry
to call get_user_pages a second time, but won't be really a retry like
in 99% of seqlock usages for the reader side, but just a second
get_user_pages to trigger a minor fault. Then if the page is different
in the second run, we'll really retry (so not in function of the
seqlock but in function of the get_user_pages page array), and there's
no risk of livelocks because get_user_pages returning a different page
won't be the common case. The seqlock should be increased first before
the invalidate and a second time once the invalidate is over.