Re: follow_page()

Andrew Morton
2004-11-11T11:06:34Z

Arjan van de Ven <arjan@infradead.org> wrote:
>
> On Thu, 2004-11-11 at 02:40 -0800, Andrew Morton wrote:
> > Can anyone think of a sane reason why this thing is marking the page dirty?
> >
> > I mean, we're supposed to mark the page dirty _after_ modifying its
> > contents.
>
> 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.