[PATCH v4] mm: introduce reference pages

[Matthew Wilcox]

On Mon, Jun 28, 2021 at 12:44:22PM -0700, John Hubbard wrote:
> On 6/28/21 12:33 PM, Matthew Wilcox wrote:
> ...
> > 
> > I wonder if single-byte captures enough of the useful possibilities.
> > In the kernel we have memset32() and memset64() [1] so we could support
> > a larger pattern than just an 8-bit byte.  It all depends what userspace
> > would find useful.
> > 
> > [1] Along with memset_p(), memset_l() and memset16() that aren't terribly
> > relevant to this use case.  Although maybe memset_l() would be the right
> > one to use since there probably aren't too many 32-bit apps that want
> > a 64-bit pattern and memset64() might not be the fastest on a 32-bit
> > kernel).
> > 
> 
> And in fact, I'm also rather intrigued by doing something like 256 copies
> of a 16-byte UUID, per 4KB page. In other words, there are *definitely*
> useful patterns that are longer than a single byte, and it seems interesting
> to support them here.
> 
> Kirill's idea of an API that somehow allows various power of 2 patterns seems
> like it would be nice, because then we don't have to pick a value that seems
> good in 2021, but less good as time goes by, perhaps.
> 
> Another thought is to use an entire 4KB page as the smallest pattern unit.
> That would allow the maximum API flexibility, because the caller could
> explicitly set every single byte in the page.

That's what this patch does.  If it can be reduced to a pattern (in
Peter's patch of a single byte; i'm proposing expanding that), then
the page is filled with the pattern; otherwise we copy the reference
page.