Re: [PATCH] Document Linux Memory Policy

[Christoph Lameter <clameter@sgi.com>]

On Thu, 31 May 2007, Gleb Natapov wrote:

> > 1. A shared range has multiple tasks that can fault pages in.
> >    The policy of which task should control how the page is allocated?
> >    Is it the last one that set the policy?

> How is it done for shmget? For my particular case I would prefer to get an error
> from numa_setlocal_memory() if process tries to set policy on the area
> of the file that already has policy set. This may happen only as a
> result of a bug in my app.

Hmmm.... Thats an idea. Lee: Do we have some way of returning an error?
We then need to have a function that clears memory policy. Maybe the
default policy is the clear?

> > 2. Pagecache pages can be read and written by buffered I/O and
> >    via mmap. Should there be different allocation semantics
> >    depending on the way you got the page? Obviously no policy
> >    for a memory range can be applied to a page allocated via
> >    buffered I/O. Later it may be mapped via mmap but then
> >    we never use policies if the page is already in memory.

> If page is already in the pagecache use it. Or return an error if strict
> policy is in use. Or something else :) In my case I make sure that files
> is accessed only through mmap interface.

On an mmap we cannot really return an error. If your program has just run 
then pages may linger in memory. If you run it on another node then the 
earlier used pages may be used.
 

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