linux-kernel - Re: [RFC][PATCHSET] sanitized pathwalk machinery (v3)

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Date:   Wed, 4 Mar 2020 16:20:51 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCHSET] sanitized pathwalk machinery (v3)

On Wed, Mar 04, 2020 at 05:28:12AM -0800, Matthew Wilcox wrote:
> On Wed, Mar 04, 2020 at 06:55:47AM +0000, Al Viro wrote:
> > On Tue, Mar 03, 2020 at 11:23:39PM -0600, Eric W. Biederman wrote:
> > > Do the xfs-tests cover that sort of thing?
> > > The emphasis is stress testing the filesystem not the VFS but there is a
> > > lot of overlap between the two.
> > 
> > I do run xfstests.  But "runs in KVM without visible slowdowns" != "won't
> > cause them on 48-core bare metal".  And this area (especially when it
> > comes to RCU mode) can be, er, interesting in that respect.
> > 
> > FWIW, I'm putting together some litmus tests for pathwalk semantics -
> > one of the things I'd like to discuss at LSF; quite a few codepaths
> > are simply not touched by anything in xfstests.
> 
> Might be more appropriate for LTP than xfstests?  will-it-scale might be
> the right place for performance benchmarks.

Might be...  I do run LTP as well, but it's still a 4-way KVM on a 6-way
amd64 host (phenom II X6 1100T) - not well-suited for catching scalability
issues.

Litmus tests mentioned above are more about verifying the semantics;
I hadn't moved past the "bunch of home-grown scripts creating setups
that would exercise the codepaths in question + trivial pieces
in C, pretty much limited to syscall()" stage with that; moving those
to LTP framework is something I'll need to look into.  Might very well
make sense; for now I just want a way to get test coverage of that code
with minimal headache.