Re: differences between MADV_FREE and MADV_DONTNEED

[Suleiman Souhlal <ssouhlal@FreeBSD.org>]

Andrea Arcangeli wrote:
> Now that MADV_REMOVE is in, should we discuss MADV_FREE?
> 
> MADV_FREE in Solaris is destructive and only works on anonymous memory,
> while MADV_DONTNEED seems to never be destructive (which I assume it
> means it's a noop on anonymous memory).

FWIW, in FreeBSD, MADV_DONTNEED is not destructive, and just makes pages 
(including anonymous ones) more likely to get swapped out.

> Our MADV_DONTNEED is destructive on anonymous memory, while it's
> non-destructive on file mappings.
> 
> Perhaps we could move the destructive anonymous part of MADV_DONTNEED to
> MADV_FREE?

This would seem like the best way to go, since it would bring Linux's 
behavior more in line with what other systems do.

> Or we could as well go relaxed and define MADV_FREE and MADV_DONTNEED
> the same way (that still leaves the question if we risk to break apps
> ported from solaris where MADV_DONTNEED is apparently always not
> destructive).
> 
> I only read the docs, I don't know in practice what MADV_DONTNEED does
> on solaris (does it return -EINVAL if run on anonymous memory or not?).
> 
> http://docs.sun.com/app/docs/doc/816-5168/6mbb3hrgk?a=view
> 
> BTW, I don't know how other specifications define MADV_FREE, but besides
> MADV_REMOVE I've also got the request to provide MADV_FREE in linux,
> this is why I'm asking. (right now I'm telling them to use #ifdef
> __linux__ #define MADV_FREE MADV_DONTNEED but that's quite an hack since
> it could break if we make MADV_DONTNEED non-destructive in the future)

FreeBSD's MADV_FREE only works on anonymous memory (it's a noop for 
vnode-backed memory), and marks the pages clean before moving them to 
the inactive queue, so that they can be freed or reused quickly, without 
causing a pagefault.

-- Suleiman