Re: differences between MADV_FREE and MADV_DONTNEED

[Christoph Hellwig <hch@infradead.org>]

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.