RE: [PATCHv3 5/6] zswap: add to mm/

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Seth Jennings [mailto:sjenning@linux.vnet.ibm.com]
> Subject: Re: [PATCHv3 5/6] zswap: add to mm/
> 
> On 01/29/2013 12:27 AM, Minchan Kim wrote:
> > First feeling is it's simple and nice approach.
> > Although we have some problems to decide policy, it could solve by later patch
> > so I hope we make basic infrasture more solid by lots of comment.
> 
> Thanks very much for the review!
> >
> > Another question.
> >
> > What's the benefit of using mempool for zsmalloc?
> > As you know, zsmalloc doesn't use mempool as default.
> > I guess you see some benefit. if so, zram could be changed.
> > If we can change zsmalloc's default scheme to use mempool,
> > all of customer of zsmalloc could be enhanced, too.
> 
> In the case of zswap, through experimentation, I found that adding a
> mempool behind the zsmalloc pool added some elasticity to the pool.
> Fewer stores failed if we kept a small reserve of pages around instead
> of having to go back to the buddy allocator who, under memory
> pressure, is more likely to reject our request.
> 
> I don't see this situation being applicable to all zsmalloc users
> however.  I don't think we want incorporate it directly into zsmalloc
> for now.  The ability to register custom page alloc/free functions at
> pool creation time allows users to do something special, like back
> with a mempool, if they want to do that.

(sorry, still catching up on backlog after being gone last week)

IIUC, by using mempool, you are essentially setting aside a
special cache of pageframes that only zswap can use (or other
users of mempool, I don't know what other subsystems use it).
So one would expect that fewer stores would fail if more
pageframes are available to zswap, the same as if you had
increased zswap_max_pool_percent by some small fraction.

But by setting those pageframes aside, you are keeping them from
general use, which may be a use with a higher priority as determined
by the mm system.

This seems wrong to me.  Should every subsystem hide a bunch of
pageframes away in case it might need them?

Or am I missing something?

Dan