Re: follow_page()

[Arjan van de Ven <arjan@infradead.org>]

> > most likely it's because the intent to write to it is given.
> > It's cheaper for the OS to mark a pagetable dirty than it's for the CPU
> > to do so (example, on a Pentium 4 it can easily take the cpu 2000 to
> > 4000 cycles to flip the dirty bit on the PTE). So if you KNOW you're
> > going to write to it (and thus the intent parameter) you can save a big
> > chunk of those cycles.
> 
> But it's racy.  writeback can write-and-clean the page before we've
> modified its contents.  Whether the page contents are altered via disk DMA
> or a memset or whatever, we can lose the data.  Except callers are
> correctly dirtying the page after modifying it anyway.

so in the race case you get the hit of those 4000 cycles... still optimizes the non-race case 
(so I'd agree that nothing should depend on this function dirtying it; it's a pre-dirty that's an
optimisation)

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"aart@kvack.org"> aart@kvack.org </a>