Re: [patch resend v4] update ctime and mtime for mmaped write

Andrew Morton
2007-03-26T22:32:01Z

On Mon, 26 Mar 2007 23:43:08 +0200
Miklos Szeredi <miklos@szeredi.hu> wrote:

> > > This patch makes writing to shared memory mappings update st_ctime and
> > > st_mtime as defined by SUSv3:
> >
> > Boy this is complicated.
>
> You tell me?
>
> > Is there a simpler way of doing all this? Say, we define a new page flag
> > PG_dirtiedbywrite and we do SetPageDirtiedByWrite() inside write() and
> > ClearPageDirtiedByWrite() whenever we propagate pte-dirtiness into
> > page-dirtiness. Then, when performing writeback we look to see if any of
> > the dirty pages are !PageDirtiedByWrite() and, if so, we update [mc]time to
> > current-time.
>
> I don't think a page flag gains anything over the address_space flag
> that this patch already has.
>
> The complexity is not about keeping track of the "data modified
> through mmap" state, but about msync() guarantees, that POSIX wants.
>
> And these requirements do in fact make some sense: msync() basically
> means:
>
> "I want the data written through mmaps to be visible to the world"
>
> And that obviously includes updating the timestamps.
>
> So how do we know if the data was modified between two msync()
> invocations? The only sane way I can think of is to walk the page
> tables in msync() and test/clear the pte dirty bit.

clear_page_dirty_for_io() already does that.

So we should be able to test PageDirtiedByWrite() after running
clear_page_dirty_for_io() to discover whether this page was dirtied via
MAP_SHARED, and then update the inode times if so.