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[2003:cb:c713:3b00:16ce:8f1c:dd50:90fb]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38c5c1cf896sm2623894f8f.94.2025.01.30.09.16.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 30 Jan 2025 09:16:46 -0800 (PST) Message-ID: <0ca9f444-b63f-4259-9462-014e0d0f1a74@redhat.com> Date: Thu, 30 Jan 2025 18:16:42 +0100 Precedence: bulk X-Mailing-List: linux-arm-msm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v2 02/11] KVM: guest_memfd: Handle final folio_put() of guest_memfd pages To: Fuad Tabba , kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org Cc: pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, seanjc@google.com, viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, yu.c.zhang@linux.intel.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@google.com, ackerleytng@google.com, mail@maciej.szmigiero.name, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, qperret@google.com, keirf@google.com, roypat@amazon.co.uk, shuah@kernel.org, hch@infradead.org, jgg@nvidia.com, rientjes@google.com, jhubbard@nvidia.com, fvdl@google.com, hughd@google.com, jthoughton@google.com References: <20250129172320.950523-1-tabba@google.com> <20250129172320.950523-3-tabba@google.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 29.01.25 18:23, Fuad Tabba wrote: > Before transitioning a guest_memfd folio to unshared, thereby > disallowing access by the host and allowing the hypervisor to > transition its view of the guest page as private, we need to be > sure that the host doesn't have any references to the folio. > > This patch introduces a new type for guest_memfd folios, which > isn't activated in this series but is here as a placeholder and > to facilitate the code in the next patch. This will be used in > the future to register a callback that informs the guest_memfd > subsystem when the last reference is dropped, therefore knowing > that the host doesn't have any remaining references. > > Signed-off-by: Fuad Tabba > --- > include/linux/page-flags.h | 7 +++++++ > mm/debug.c | 1 + > mm/swap.c | 5 +++++ > 3 files changed, 13 insertions(+) > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > index 6615f2f59144..bab3cac1f93b 100644 > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -942,6 +942,7 @@ enum pagetype { > PGTY_slab = 0xf5, > PGTY_zsmalloc = 0xf6, > PGTY_unaccepted = 0xf7, > + PGTY_guestmem = 0xf8, > > PGTY_mapcount_underflow = 0xff > }; > @@ -1091,6 +1092,12 @@ FOLIO_TYPE_OPS(hugetlb, hugetlb) > FOLIO_TEST_FLAG_FALSE(hugetlb) > #endif > Some short doc would be nice, to at least hint that this is related to guest_memfd, and that these are otherwise folios. /* * guestmem folios are folios that are used to back VM memory as managed * guest_memfd. Once the last reference is put, instead of freeing these * folios back to the page allocator, they are returned to guest_memfd. * * For now, guestmem will only be set on these folios as long as they * cannot be mapped to user space ("private state"), with the plan of * always setting that type once typed folios can be mapped to user * space cleanly. */ > +#ifdef CONFIG_KVM_GMEM_MAPPABLE > +FOLIO_TYPE_OPS(guestmem, guestmem) > +#else > +FOLIO_TEST_FLAG_FALSE(guestmem) > +#endif > + > PAGE_TYPE_OPS(Zsmalloc, zsmalloc, zsmalloc) > > /* > diff --git a/mm/debug.c b/mm/debug.c > index 95b6ab809c0e..db93be385ed9 100644 > --- a/mm/debug.c > +++ b/mm/debug.c > @@ -56,6 +56,7 @@ static const char *page_type_names[] = { > DEF_PAGETYPE_NAME(table), > DEF_PAGETYPE_NAME(buddy), > DEF_PAGETYPE_NAME(unaccepted), > + DEF_PAGETYPE_NAME(guestmem), > };> > static const char *page_type_name(unsigned int page_type) > diff --git a/mm/swap.c b/mm/swap.c > index 8a66cd9cb9da..73d61c7f8edd 100644 > --- a/mm/swap.c > +++ b/mm/swap.c > @@ -37,6 +37,7 @@ > #include > #include > #include > +#include > > #include "internal.h" > > @@ -101,6 +102,10 @@ static void free_typed_folio(struct folio *folio) > if (IS_ENABLED(CONFIG_HUGETLBFS)) > free_huge_folio(folio); > return; > + case PGTY_guestmem: > + if (IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM)) > + WARN_ONCE(1, "A placeholder that shouldn't trigger."); Does it make sense to directly introduce the callback into guest_memfd and handle the WARN_ONCE() in there? Then, we don't have tot ouch this core code later again. -- Cheers, David / dhildenb