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Date:   Tue, 30 Oct 2018 15:08:59 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Igor Stoppa <igor.stoppa@gmail.com>
Cc:     Andy Lutomirski <luto@amacapital.net>, nadav.amit@gmail.com,
        Kees Cook <keescook@chromium.org>,
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        James Morris <jmorris@namei.org>,
        Michal Hocko <mhocko@kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-integrity <linux-integrity@vger.kernel.org>,
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        Igor Stoppa <igor.stoppa@huawei.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Laura Abbott <labbott@redhat.com>,
        Randy Dunlap <rdunlap@infradead.org>,
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        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
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Subject: Re: [PATCH 10/17] prmem: documentation
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On Tue, Oct 30, 2018 at 11:55:46PM +0200, Igor Stoppa wrote:
> On 30/10/2018 23:25, Matthew Wilcox wrote:
> > On Tue, Oct 30, 2018 at 11:51:17AM -0700, Andy Lutomirski wrote:
> > > Finally, one issue: rare_alloc() is going to utterly suck
> > > performance-wise due to the global IPI when the region gets zapped out
> > > of the direct map or otherwise made RO.  This is the same issue that
> > > makes all existing XPO efforts so painful. We need to either optimize
> > > the crap out of it somehow or we need to make sure it’s not called
> > > except during rare events like device enumeration.
> > 
> > Batching operations is kind of the whole point of the VM ;-)  Either
> > this rare memory gets used a lot, in which case we'll want to create slab
> > caches for it, make it a MM zone and the whole nine yeards, or it's not
> > used very much in which case it doesn't matter that performance sucks.
> > 
> > For now, I'd suggest allocating 2MB chunks as needed, and having a
> > shrinker to hand back any unused pieces.
> 
> One of the prime candidates for this sort of protection is IMA.
> In the IMA case, there are ever-growing lists which are populated when
> accessing files.
> It's something that ends up on the critical path of any usual performance
> critical use case, when accessing files for the first time, like at
> boot/application startup.
> 
> Also the SELinux AVC is based on lists. It uses an object cache, but it is
> still something that grows and is on the critical path of evaluating the
> callbacks from the LSM hooks. A lot of them.
> 
> These are the main two reasons, so far, for me advocating an optimization of
> the write-rare version of the (h)list.

I think these are both great examples of why doubly-linked lists _suck_.
You have to modify three cachelines to add an entry to a list.  Walking a
linked list is an exercise in cache misses.  Far better to use an XArray /
IDR for this purpose.