Re: 2.6.22 -mm merge plans -- vm bugfixes

[Nick Piggin <nickpiggin@yahoo.com.au>]

Hugh Dickins wrote:
> On Tue, 1 May 2007, Nick Piggin wrote:

> > There were concerns that we could do this more cheaply, but I think it
> > is important to start with a base that is simple and more likely to
> > be correct and build on that. My testing didn't show any obvious
> > problems with performance.
>
>
> I don't see _problems_ with performance, but I do consistently see the
> same kind of ~5% degradation in lmbench fork, exec, sh, mmap latency
> and page fault tests on SMP, several machines, just as I did last year.

OK. I did run some tests at one stage which didn't show a regression
on my P4, however I don't know that they were statistically significant.
I'll try a couple more runs and post numbers.


> I'm assuming this patch is the one responsible: at 2.6.20-rc4 time
> you posted a set of 10 and a set of 7 patches I tried in versus out;
> at 2.6.21-rc3-mm2 time you had a group of patches in -mm I tried in
> versus out; with similar results.
>
> I did check the graphs on test.kernel.org, I couldn't see any bad
> behaviour there that correlated with this work; though each -mm
> has such a variety of new work in it, it's very hard to attribute.
> And nobody else has reported any regression from your patches.
>
> I'm inclined to write it off as poorer performance in some micro-
> benchmarks, against which we offset the improved understandabilty
> of holding the page lock over the file fault.
>
> But I was quite disappointed when 
> mm-fix-fault-vs-invalidate-race-for-linear-mappings-fix.patch
> appeared, putting double unmap_mapping_range calls in.  Certainly
> you were wrong to take the one out, but a pity to end up with two.
>
> Your comment says/said:
> The nopage vs invalidate race fix patch did not take care of truncating
> private COW pages. Mind you, I'm pretty sure this was previously racy
> even for regular truncate, not to mention vmtruncate_range.
>
> vmtruncate_range (holepunch) was deficient I agree, and though we
> can now take out your second unmap_mapping_range there, that's only
> because I've slipped one into shmem_truncate_range.  In due course it
> needs to be properly handled by noting the range in shmem inode info.
>
> (I think you couldn't take that approach, noting invalid range in
> ->mapping while invalidating, because NFS has/had some cases of
> invalidate_whatever without i_mutex?)

Sorry, I didn't parse this? But I wonder whether it is better to do
it in vmtruncate_range than the filesystem? Private COWed pages are
not really a filesystem "thing"...


> But I'm pretty sure (to use your words!) regular truncate was not racy
> before: I believe Andrea's sequence count was handling that case fine,
> without a second unmap_mapping_range.

OK, I think you're right. I _think_ it should also be OK with the
lock_page version as well: we should not be able to have any pages
after the first unmap_mapping_range call, because of the i_size
write. So if we have no pages, there is nothing to 'cow' from.


> Well, I guess I've come to accept that, expensive as unmap_mapping_range
> may be, truncating files while they're mmap'ed is perverse behaviour:
> perhaps even deserving such punishment.
>
> But it is a shame, and leaves me wondering what you gained with the
> page lock there.
>
> One thing gained is ease of understanding, and if your later patches
> build an edifice upon the knowledge of holding that page lock while
> faulting, I've no wish to undermine that foundation.

It also fixes a bug, doesn't it? ;)

--
SUSE Labs, Novell Inc.
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