Re: [PATCH] remove throttle_vm_writeout()

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

On Fri, 05 Oct 2007 01:26:12 +0200
Miklos Szeredi <miklos@szeredi.hu> wrote:

> > This is a somewhat general problem: a userspace process is in the IO path. 
> > Userspace block drivers, for example - pretty much anything which involves
> > kernel->userspace upcalls for storage applications.
> > 
> > I solved it once in the past by marking the userspace process as
> > PF_MEMALLOC and I beleive that others have implemented the same hack.
> > 
> > I suspect that what we need is a general solution, and that the solution
> > will involve explicitly telling the kernel that this process is one which
> > actually cleans memory and needs special treatment.
> > 
> > Because I bet there will be other corner-cases where such a process needs
> > kernel help, and there might be optimisation opportunities as well.
> > 
> > Problem is, any such mark-me-as-special syscall would need to be
> > privileged, and FUSE servers presently don't require special perms (do
> > they?)
> 
> No, and that's a rather important feature, that I'd rather not give
> up.

Can fuse do it?  Perhaps the fs can diddle the server's task_struct at
registration time?

>  But with the dirty limiting, the memory cleaning really shouldn't
> be a problem, as there is plenty of memory _not_ used for dirty file
> data, that the filesystem can use during the writeback.

I don't think I understand that.  Sure, it _shouldn't_ be a problem.  But it
_is_.  That's what we're trying to fix, isn't it?

> So the only thing the kernel should be careful about, is not to block
> on an allocation if not strictly necessary.
> 
> Actually a trivial fix for this problem could be to just tweak the
> thresholds, so to make the above scenario impossible.  Although I'm
> still not convinced, this patch is perfect, because the dirty
> threshold can actually change in time...
> 
> Index: linux/mm/page-writeback.c
> ===================================================================
> --- linux.orig/mm/page-writeback.c      2007-10-05 00:31:01.000000000 +0200
> +++ linux/mm/page-writeback.c   2007-10-05 00:50:11.000000000 +0200
> @@ -515,6 +515,12 @@ void throttle_vm_writeout(gfp_t gfp_mask
>          for ( ; ; ) {
>                 get_dirty_limits(&background_thresh, &dirty_thresh, NULL, NULL);
> 
> +               /*
> +                * Make sure the theshold is over the hard limit of
> +                * dirty_thresh + ratelimit_pages * nr_cpus
> +                */
> +               dirty_thresh += ratelimit_pages * num_online_cpus();
> +
>                  /*
>                   * Boost the allowable dirty threshold a bit for page
>                   * allocators so they don't get DoS'ed by heavy writers

I can probably kind of guess what you're trying to do here.  But if
ratelimit_pages * num_online_cpus() exceeds the size of the offending zone
then things might go bad.