Date: Wed, 31 Oct 2018 10:29:42 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Matthew Wilcox <willy@...radead.org> Cc: Andy Lutomirski <luto@...capital.net>, nadav.amit@...il.com, Kees Cook <keescook@...omium.org>, Igor Stoppa <igor.stoppa@...il.com>, Mimi Zohar <zohar@...ux.vnet.ibm.com>, Dave Chinner <david@...morbit.com>, James Morris <jmorris@...ei.org>, Michal Hocko <mhocko@...nel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, linux-integrity <linux-integrity@...r.kernel.org>, linux-security-module <linux-security-module@...r.kernel.org>, Igor Stoppa <igor.stoppa@...wei.com>, Dave Hansen <dave.hansen@...ux.intel.com>, Jonathan Corbet <corbet@....net>, Laura Abbott <labbott@...hat.com>, Randy Dunlap <rdunlap@...radead.org>, Mike Rapoport <rppt@...ux.vnet.ibm.com>, "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de> Subject: Re: [PATCH 10/17] prmem: documentation On Tue, Oct 30, 2018 at 02:25:51PM -0700, 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. Yes, for the dynamic case something along those lines would be needed. If we have a single rare zone, we could even have __GFP_RARE or whatever that manages this. The page allocator would have to grow a rare memblock type, and every rare alloc would allocate from a rare memblock, when none is available, creation of a rare block would set up the mappings etc.. > For now, I'd suggest allocating 2MB chunks as needed, and having a > shrinker to hand back any unused pieces. Something like the percpu allocator?
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