Re: [patch 2/8] update ctime and mtime for mmaped write

Miklos Szeredi
2007-03-06T22:01:49Z

> > > I can understand you wanting to avoid the overhead of the minor faults
> > > resulting from using page_mkclean(), but I'm not sure its worth it.
> >
> > It would be nice if the cost of MS_ASYNC wouldn't be too high. And I
> > do have the feeling that minor faults are far more expensive than
> > cleaning the dirty bit in the ptes.
> >
> > Do you have any numbers?
>
> None what so ever, but I always think of msync as a rare function
> (infrequent when compared to (minor) faults overall). But I don't have
> numbers backing that up either.

It depends entirely on the usage pattern. I can imagine this sort of
use:

mmap
write lots of data to memory
msync(MS_ASYNC)
overwrite previous data
msync(MS_ASYNC)
...

In this case write protecting and faulting the pages will be slower,
than just checking the page tables.

> Also, the radix tree scan you do isn't exactly cheap either.
>
> So what I was wondering is whether its worth optimizing this at the cost
> of another rmap walker. (one with very dubious semantics at that - it
> clears the pte dirty bit but doesn't particularly care about that nor
> does it respect the PG_dirty / PTE dirty relation)

I don't think this is dubious. The pte dirty bit in this case means,
that the page was modified _since_the_last_call_ of this function.

The PG_dirty on the other hand means, that the page will need to be
written back. So they have completely different meanings.

Thanks,
Miklos