Peter Zijlstra wrote:

>People expressed the need to track dirty pages in shared mappings.
>Linus outlined the general idea of doing that through making clean
>writable pages write-protected and taking the write fault.
>
>This patch does exactly that, it makes pages in a shared writable
>mapping write-protected. On write-fault the pages are marked dirty and
>made writable. When the pages get synced with their backing store, the
>write-protection is re-instated.
>
>It survives a simple test and shows the dirty pages in /proc/vmstat.
>
>Comments?
>

Looks pretty good. Christoph and I were looking at ways to improve
performance impact of this, and skipping the extra work for particular
(eg. shmem) mappings might be a good idea?

Attached is a patch with a couple of things I've currently got.

In the long run, I'd like to be able to set_page_dirty and
balance_dirty_pages outside of both ptl and mmap_sem, for performance
reasons. That will require a reworking of arch code though :(