linux-kernel - Re: [vfs] 8bb3c61baf: vm-scalability.median -23.7% regression

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Date:   Mon, 9 Sep 2019 23:27:15 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
cc:     Hugh Dickins <hughd@...gle.com>,
        kernel test robot <rong.a.chen@...el.com>,
        David Howells <dhowells@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, lkp@...org
Subject: Re: [vfs]  8bb3c61baf:  vm-scalability.median -23.7% regression

On Mon, 9 Sep 2019, Al Viro wrote:
> 
> Anyway, see vfs.git#uncertain.shmem for what I've got with those folded in.
> Do you see any problems with that one?  That's the last 5 commits in there...

It's mostly fine, I've no problem with going your way instead of what
we had in mmotm; but I have seen some problems with it, and had been
intending to send you a fixup patch tonight (shmem_reconfigure() missing
unlock on error is the main problem, but there are other fixes needed).

But I'm growing tired. I've a feeling my "swap" of the mpols, instead
of immediate mpol_put(), was necessary to protect against a race with
shmem_get_sbmpol(), but I'm not clear-headed enough to trust myself on
that now.  And I've a mystery to solve, that shmem_reconfigure() gets
stuck into showing the wrong error message.

Tomorrow....

Oh, and my first attempt to build and boot that series over 5.3-rc5
wouldn't boot. Luckily there was a tell-tale "i915" in the stacktrace,
which reminded me of the drivers/gpu/drm/i915/gem/i915_gemfs.c fix
we discussed earlier in the cycle.  That is of course in linux-next
by now, but I wonder if your branch ought to contain a duplicate of
that fix, so that people with i915 doing bisections on 5.4-rc do not
fall into an unbootable hole between vfs and gpu merges.

Hugh