linux-kernel - Re: [RFC PATCH 0/5] Prototype for direct map awareness in page allocator

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Message-ID: <ZGeYgKCFOkzP2fub@google.com>
Date:   Fri, 19 May 2023 08:40:48 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Rick P Edgecombe <rick.p.edgecombe@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "song@...nel.org" <song@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "x86@...nel.org" <x86@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 0/5] Prototype for direct map awareness in page allocator

On Thu, Mar 09, 2023, Mike Rapoport wrote:
> On Thu, Mar 09, 2023 at 01:59:00AM +0000, Edgecombe, Rick P wrote:
> > On Wed, 2023-03-08 at 11:41 +0200, Mike Rapoport wrote:
> > > From: "Mike Rapoport (IBM)" <rppt@...nel.org>
> > > 
> > > Hi,
> > > 
> > > This is a third attempt to make page allocator aware of the direct
> > > map
> > > layout and allow grouping of the pages that must be unmapped from
> > > the direct map.
> > > 
> > > This a new implementation of __GFP_UNMAPPED, kinda a follow up for
> > > this set:
> > > 
> > > https://lore.kernel.org/all/20220127085608.306306-1-rppt@kernel.org
> > > 
> > > but instead of using a migrate type to cache the unmapped pages, the
> > > current implementation adds a dedicated cache to serve __GFP_UNMAPPED
> > > allocations.
> > 
> > It seems a downside to having a page allocator outside of _the_ page
> > allocator is you don't get all of the features that are baked in there.
> > For example does secretmem care about numa? I guess in this
> > implementation there is just one big cache for all nodes.
> > 
> > Probably most users would want __GFP_ZERO. Would secretmem care about
> > __GFP_ACCOUNT?
> 
> The intention was that the pages in cache are always zeroed, so __GFP_ZERO
> is always implicitly there, at least should have been.

Would it be possible to drop that assumption/requirement, i.e. allow allocation of
__GFP_UNMAPPED without __GFP_ZERO?  At a glance, __GFP_UNMAPPED looks like it would
be a great fit for backing guest memory, in particular for confidential VMs.  And
for some flavors of CoCo, i.e. TDX, the trusted intermediary is responsible for
zeroing/initializing guest memory as the untrusted host (kernel/KVM) doesn't have
access to the guest's encryption key.  In other words, zeroing in the kernel would
be unnecessary work.