Re: differences between MADV_FREE and MADV_DONTNEED

[Nicholas Miell <nmiell@comcast.net>]

On Tue, 2006-01-17 at 11:23 -0700, Eric W. Biederman wrote:
> Christoph Hellwig <hch@infradead.org> writes:
> > On Mon, Jan 16, 2006 at 05:04:07PM -0800, Nicholas Miell wrote:
> >> On Mon, 2006-01-16 at 16:24 -0800, Suleiman Souhlal wrote:
> >> > Well, imho, MADV_DONTNEED should mean "I won't need this anytime soon", 
> >> > and MADV_FREE "I will never need this again".
> >> > 
> >> 
> >> POSIX doesn't have a madvise(), but it does have a posix_madvise(), with
> >> flags defined as follows:
> >> 
> >> POSIX_MADV_NORMAL
> >>    Specifies that the application has no advice to give on its behavior
> >> with respect to the specified range. It is the default characteristic if
> >> no advice is given for a range of memory.
> >> POSIX_MADV_SEQUENTIAL
> >>    Specifies that the application expects to access the specified range
> >> sequentially from lower addresses to higher addresses.
> >> POSIX_MADV_RANDOM
> >>    Specifies that the application expects to access the specified range
> >> in a random order.
> >> POSIX_MADV_WILLNEED
> >>    Specifies that the application expects to access the specified range
> >> in the near future.
> >> POSIX_MADV_DONTNEED
> >>    Specifies that the application expects that it will not access the
> >> specified range in the near future.
> >> 
> >> Note that glibc forwards posix_madvise() directly to madvise(2), which
> >> means that right now, POSIX conformant apps which use
> >> posix_madvise(addr, len, POSIX_MADV_DONTNEED) are silently corrupting
> >> data on Linux systems.
> >
> > Does our MAD_DONTNEED numerical value match glibc's POSIX_MADV_DONTNEED?
> >
> > In either case I'd say we should backout this patch for now.  We should
> > implement a real MADV_DONTNEED and rename the current one to MADV_FREE,
> > but that's 2.6.17 material.
> 
> We definitely need to check this.  I am fairly certain  I have seen this conversation
> before.

Yes, POSIX_MADV_* have the same values as MADV_*. And if you're trying
to find the actual implementation of posix_madvise() to verify its
behavior, it is generated by script from a line in
libc/sysdeps/unix/sysv/linux/syscalls.list.

-- 
Nicholas Miell <nmiell@comcast.net>