Re: differences between MADV_FREE and MADV_DONTNEED

[Badari Pulavarty <pbadari@us.ibm.com>]

On Tue, 2006-01-17 at 12:43 +0000, Christoph Hellwig wrote:
> 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:
> > > Eric W. Biederman wrote:
> > > > As I recall the logic with DONTNEED was to mark the mapping of
> > > > the page clean so the page didn't need to be swapped out, it could
> > > > just be dropped.
> > > > 
> > > > That is why they anonymous and the file backed cases differ.
> > > > 
> > > > Part of the point is to avoid the case of swapping the pages out if
> > > > the application doesn't care what is on them anymore.
> > > 
> > > 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.

Christoph,

What patch are you recommending backing out ? 

Thanks,
Badari

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