Messages in this thread | | | Subject | Re: mmap dirty limits on 32 bit kernels (Was: [BUG] New Kernel Bugs) | From | Peter Zijlstra <> | Date | Thu, 15 Nov 2007 22:26:54 +0100 |
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On Thu, 2007-11-15 at 13:14 -0800, Linus Torvalds wrote: > > On Thu, 15 Nov 2007, Linus Torvalds wrote: > > > > Unacceptable. We used to do exactly what your patch does, and it got fixed > > once. We're not introducing that fundamentally broken concept again. > > Examples of non-broken solutions: > (a) always use lowmem sizes (what we do now) > (b) always use total mem sizes (sane but potentially dangerous: but the > VM pressure should work! It has serious bounce-buffer issues, though, > which is why I think it's crazy even if it's otherwise consistent) > (c) make all dirty counting be *purely* per-bdi, so that everybody can > disagree on what the limits are, but at least they also then use > different counters
I think that (c) is doable. If its worth the effort, who knows, apparently there still are people using 32bit kernels on boxen with mucho memory.
> So it's just the "different writers look at the same dirty counts but then > interpret it to mean totally different things" that I think is so > fundamentally bogus. I'm not claiming that what we do now is the only way > to do things, I just don't think your approach is tenable.
Agreed, the per mapping thing was utter crap.
> I'd also like to point out that while the "bounce buffer" issue is not so > much a HIGHMEM issue on its own (it's really about the device DMA limits, > which are _independent_ of HIGHMEM, of course), the reason HIGHMEM is > special is that without HIGHMEM the bounce buffers generally work > perfectly fine. > > The problem with HIGHMEM is that it causes various metadata (dentries, > inodes, page struct tables etc) to eat up memory "prime real estate" under > the same kind of conditions that also dirty a lot of memory. So the reason > we disallow HIGHMEM from dirty limits is only *partly* the per-device or > mapping DMA limits, and to a large degree the fact that non-highmem memory > is special in general, and it is usually the non-highmem areas that are > constrained - and need to be protected.
But this problem is already an issue, Anton recently had a case where a 12GB highmem box locked up due to NTFS running out of lowmem - or something like that.
And I think that with the targeted slab reclaim (or slab defrag as its apparently still called) we can properly fix this side of the problem. I think Rik was looking into doing so.
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