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[2003:cb:c745:2000:5e9f:9789:2c3b:8b3d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43d1fe2a2c8sm11664425e9.23.2025.03.14.02.33.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 14 Mar 2025 02:33:08 -0700 (PDT) Message-ID: <18db10a0-bd40-4c6a-b099-236f4dcaf0cf@redhat.com> Date: Fri, 14 Mar 2025 10:33:07 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC v1 0/5] KVM: gmem: 2MB THP support and preparedness tracking changes To: Yan Zhao Cc: "Shah, Amit" , "kvm@vger.kernel.org" , "Roth, Michael" , "liam.merwick@oracle.com" , "seanjc@google.com" , "jroedel@suse.de" , "linux-mm@kvack.org" , "Sampat, Pratik Rajesh" , "linux-kernel@vger.kernel.org" , "Lendacky, Thomas" , "vbabka@suse.cz" , "pbonzini@redhat.com" , "linux-coco@lists.linux.dev" , "quic_eberman@quicinc.com" , "Kalra, Ashish" , "ackerleytng@google.com" , "vannapurve@google.com" References: <20241212063635.712877-1-michael.roth@amd.com> <11280705-bcb1-4a5e-a689-b8a5f8a0a9a6@redhat.com> <3bd7936624b11f755608b1c51cc1376ebf2c3a4f.camel@amd.com> <6e55db63-debf-41e6-941e-04690024d591@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: C4C8640004 X-Rspamd-Server: rspam03 X-Stat-Signature: krf3p7cjedr3dgtneqm1rqwucwpi4tem X-HE-Tag: 1741944793-251215 X-HE-Meta: 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 TtnfjR8Z Nk8uMclilqyNgiXCpjA6/oCZ3lvzed7XHVkN7jK1QkObyo4fh9cjHWi/PQpUTmYSfiQPonBzucE5lA7VooKe/Nxw8lCRmHgqGhDrEa749T+XfRBVnLWTz7zflxcHGXhPbddcKcQhmFHuGXNcS+EmMwvWx7gKSVQHX5+nRBeYqIp6yQt1TYbAxlcIuOX52/nXX5MS6li3y8ajAOFjgghYjzkH+7YaX0+wFQ8cNBl/S4lyCQMWph8xhmfmtridPTqq8AqJBWUjRiNxul8vNbdx5xatdP6r7TGnLCs2JUCzvAnaysNLpZVoNCHtB0/OFNj6TE6kmTAlk9a0vgbajiFn+HlowD4v3uom+QHyfbvhg/yGop+9mxw6K2m/dYvDWs3PORxNJ/2MO4ptRsKNTGXK5uDBedUXt8S8rOtDvXocKuvEgAZwyp9jFRdf4vQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 14.03.25 10:09, Yan Zhao wrote: > On Wed, Jan 22, 2025 at 03:25:29PM +0100, David Hildenbrand wrote: >> (split is possible if there are no unexpected folio references; private >> pages cannot be GUP'ed, so it is feasible) > ... >>>> Note that I'm not quite sure about the "2MB" interface, should it be >>>> a >>>> "PMD-size" interface? >>> >>> I think Mike and I touched upon this aspect too - and I may be >>> misremembering - Mike suggested getting 1M, 2M, and bigger page sizes >>> in increments -- and then fitting in PMD sizes when we've had enough of >>> those. That is to say he didn't want to preclude it, or gate the PMD >>> work on enabling all sizes first. >> >> Starting with 2M is reasonable for now. The real question is how we want to >> deal with > Hi David, > Hi! > I'm just trying to understand the background of in-place conversion. > > Regarding to the two issues you mentioned with THP and non-in-place-conversion, > I have some questions (still based on starting with 2M): > >> (a) Not being able to allocate a 2M folio reliably > If we start with fault in private pages from guest_memfd (not in page pool way) > and shared pages anonymously, is it correct to say that this is only a concern > when memory is under pressure? Usually, fragmentation starts being a problem under memory pressure, and memory pressure can show up simply because the page cache makes us of as much memory as it wants. As soon as we start allocating a 2 MB page for guest_memfd, to then split it up + free only some parts back to the buddy (on private->shared conversion), we create fragmentation that cannot get resolved as long as the remaining private pages are not freed. A new conversion from shared->private on the previously freed parts will allocate other unmovable pages (not the freed ones) and make fragmentation worse. In-place conversion improves that quite a lot, because guest_memfd tself will not cause unmovable fragmentation. Of course, under memory pressure, when and cannot allocate a 2M page for guest_memfd, it's unavoidable. But then, we already had fragmentation (and did not really cause any new one). We discussed in the upstream call, that if guest_memfd (primarily) only allocates 2M pages and frees 2M pages, it will not cause fragmentation itself, which is pretty nice. > >> (b) Partial discarding > For shared pages, page migration and folio split are possible for shared THP? I assume by "shared" you mean "not guest_memfd, but some other memory we use as an overlay" -- so no in-place conversion. Yes, that should be possible as long as nothing else prevents migration/split (e.g., longterm pinning) > > For private pages, as you pointed out earlier, if we can ensure there are no > unexpected folio references for private memory, splitting a private huge folio > should succeed. Yes, and maybe (hopefully) we'll reach a point where private parts will not have a refcount at all (initially, frozen refcount, discussed during the last upstream call). Are you concerned about the memory fragmentation after repeated > partial conversions of private pages to and from shared? Not only repeated, even just a single partial conversion. But of course, repeated partial conversions will make it worse (e.g., never getting a private huge page back when there was a partial conversion). -- Cheers, David / dhildenb