> This is a somewhat general problem: a userspace process is in the IO path. 
quoted text
> 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.  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.

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


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