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AHgh+RqpYHPlYGLfIIqlhdS0Wx6uVHOPLt7M3RVjcI2w/4Jok7BvHIlYq1iMiUW38E7h2ZarEJh/0oKjaS1mmdQ=@vger.kernel.org X-Gm-Message-State: AOJu0YzXhH+geCW33e8Qsr0bVWY7g2o8Qs/r/3cVQ5IrYAexsGqWCTaS QLGVkKhVT/lQnq8LnlIZptkhyXiabHo3W1YyzJ6U8d6P5Kvw6jvIx0myv3I/Eff/vDrWaPDFfBk c2LPRpHJrrnOUSj3rJA== X-Received: from edbek14.prod.google.com ([2002:a05:6402:370e:b0:69f:da3d:dc37]) (user=tarunsahu job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6402:5048:b0:69a:a4cb:2882 with SMTP id 4fb4d7f45d1cf-6a355db411emr1046692a12.9.1786367331218; Mon, 10 Aug 2026 06:08:51 -0700 (PDT) Date: Mon, 10 Aug 2026 13:08:50 +0000 In-Reply-To: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260728121138.1103610-1-tarunsahu@google.com> <20260728121138.1103610-8-tarunsahu@google.com> Message-ID: <9huz7bly86n1.fsf@tarunix.c.googlers.com> Subject: Re: [PATCH v4 07/11] KVM: guest_memfd: Add support for freezing mappings From: tarunsahu@google.com To: Ackerley Tng , fuad.tabba@linux.dev, Andrew Morton , seanjc@google.com, dmatlack@google.com, Shuah Khan , Jonathan Corbet , david@redhat.com, Pasha Tatashin , Pratyush Yadav , sagis@google.com, Paolo Bonzini , Mike Rapoport , Alexander Graf Cc: linux-kselftest@vger.kernel.org, andre.przywara@arm.com, michael.roth@amd.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, will@kernel.org, vannapurve@google.com, maz@kernel.org, fvdl@google.com, kvm@vger.kernel.org, oliver.upton@linux.dev, kvmarm@lists.linux.dev, alexandru.elisei@arm.com, skhawaja@google.com, aneesh.kumar@kernel.org, linux-doc@vger.kernel.org, David Hildenbrand , yan.y.zhao@intel.com, kexec@lists.infradead.org, suzuki.poulose@arm.com Content-Type: text/plain; charset="UTF-8" Ackerley Tng writes: > Tarun Sahu writes: > >> >> [...snip...] >> >> @@ -273,16 +284,30 @@ static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len) >> static long kvm_gmem_fallocate(struct file *file, int mode, loff_t offset, >> loff_t len) >> { >> + struct inode *inode = file_inode(file); >> int ret; >> + int idx; >> >> - if (!(mode & FALLOC_FL_KEEP_SIZE)) >> - return -EOPNOTSUPP; >> + idx = srcu_read_lock(&kvm_gmem_freeze_srcu); >> + if (kvm_gmem_is_frozen(inode)) { >> + srcu_read_unlock(&kvm_gmem_freeze_srcu, idx); >> + return -EPERM; >> + } >> >> - if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE)) >> - return -EOPNOTSUPP; >> + if (!(mode & FALLOC_FL_KEEP_SIZE)) { >> + ret = -EOPNOTSUPP; >> + goto out; >> + } >> >> - if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len)) >> - return -EINVAL; >> + if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE)) { >> + ret = -EOPNOTSUPP; >> + goto out; >> + } >> + >> + if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len)) { >> + ret = -EINVAL; >> + goto out; >> + } >> > > Continuing from [1], this guard is actually meant for PUNCH_HOLE... I > wonder if there could be a deeper refactoring to check when actually > punching a hole, basically the equivalent place as kvm_gmem_get_folio() > in the flow of fallocating. > > Will srcu_read_lock() be taken in a nested way for the fallocate case? > It can be taken in a nested way with some intricacies, but maybe if the > freeze check can be done just as the folio is about to be removed (as > described in the above paragraph), it would avoid this nested locking > here. > > [1] https://lore.kernel.org/all/2vxzzf0kl3dt.fsf@kernel.org/ Instead of avoiding on remove folio, We can just add the freeze check across the PUNCH_HOLE, So there will be no nested lock anymore. Nested lock is just with fallocate populate -> kvm_gmem_get_pfn. So this nested ness will be removed with this update. ~Tarun > >> if (mode & FALLOC_FL_PUNCH_HOLE) >> ret = kvm_gmem_punch_hole(file_inode(file), offset, len); >> @@ -291,6 +316,9 @@ static long kvm_gmem_fallocate(struct file *file, int mode, loff_t offset, >> >> if (!ret) >> file_modified(file); >> + >> +out: >> + srcu_read_unlock(&kvm_gmem_freeze_srcu, idx); >> return ret; >> } >> >> >> [...snip...] >>