Re: [RFC 1/2] deactive invalidated pages

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 23 Nov 2010 14:58:56 +0000
Mel Gorman <mel@csn.ul.ie> wrote:

> On Tue, Nov 23, 2010 at 09:55:49AM -0500, Ben Gamari wrote:
> > On Tue, 23 Nov 2010 09:38:59 +0000, Mel Gorman <mel@csn.ul.ie> wrote:
> > > > If it's mapped pagecache then the user was being a bit silly (or didn't
> > > > know that some other process had mapped the file).  In which case we
> > > > need to decide what to do - leave the page alone, deactivate it, or
> > > > half-deactivate it as this patch does.
> > > > 
> > > 
> > > What are the odds of an fadvise() user having used mincore() in advance
> > > to determine if the page was in use by another process? I would guess
> > > "low" so this half-deactivate gives a chance for the page to be promoted
> > > again as well as a chance for the flusher threads to clean the page if
> > > it really is to be reclaimed.
> > > 
> > Do we really want to make the user jump through such hoops as using
> > mincore() just to get the kernel to handle use-once pages properly?
> 
> I would think "no" which is why I support half-deactivating pages so they won't
> have to.

If the page is page_mapped() then we can assume that some other process
is using it and we leave it alone *altogether*.

If the page is dirty or under writeback (and !page_mapped()) then we
should assume that we should free it asap.  The PageReclaim() trick
might help with that.

I just don't see any argument for moving the page to the head of the
inactive LRU as a matter of policy.  We can park it there because we
can't think of anythnig else to do with it, but it's the wrong place
for it.