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b=V7+HQvRwM9bPb0Rxkkv5aHO99w3TUgqWDsppu0UG6MQTNDMsPwzNKNIG2rQPEJ6LX 1tVgVAI/NI6k2O3UX6ZxwITvnJwIN794QtGOMz6rSEu6I4kSc68W+gn1B9MzhmSUjK lxUbOGRzV0jEHIwzfhiCpwaV4slLgVJYSJeE7Rqygrv0lA8tEw7Rk6Vt4ZQH7cgKP7 yYXhPFFickcwkFwVw9Omcj7Ze/yFW1kNeeVLq8WuQN/NfkM55kr9v7Bveup3cyx/il cX6zxqy072LUeqbhFrMk72NeBYauAgvZVTdERArq2xQcocEJ4gheOB475fPltfsFzH v6g3P8mdHQibg== Message-ID: Date: Mon, 23 Feb 2026 16:23:21 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 00/10] guest_memfd: Track amount of memory allocated on inode To: Ackerley Tng , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org Cc: akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, willy@infradead.org, pbonzini@redhat.com, shuah@kernel.org, seanjc@google.com, shivankg@amd.com, rick.p.edgecombe@intel.com, yan.y.zhao@intel.com, rientjes@google.com, fvdl@google.com, jthoughton@google.com, vannapurve@google.com, pratyush@kernel.org, pasha.tatashin@soleen.com, kalyazin@amazon.com, tabba@google.com, michael.roth@amd.com References: From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: E94E64000F X-Stat-Signature: 5yrnz37qgrw96pbz5594qfd6astwsjmp X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1771860211-85254 X-HE-Meta: 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 9ifWHmmV Y4gNpgprLFnw5oFGRLiZov7MpSy2x/APhNnvh7+/wWzRr9LUrowybcDihagtGj5F4i+WG7PpV6tY0OZuGeqpVpk90h/U4EH7A5dxWafv5iQwb1aQMw2lbaaLz2vqC8nq42Fdws376EfF7j9STKZH/+WFWIY3h8BkR+D4R7YC7BJfeTY0Ic9CbRiEpk+3nBSzrAva4Ne6qy7Jse34Jv23+09Q0Me683NvVkXk8PYaY7iKqG2YyXpPjBH5zOzXXTnNFBydqJP9ZYindOPrVhWqHNVJ22AZPFDDM+CK/3Dvfd46qSilvMmcOQMYHFw== 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 2/23/26 08:04, Ackerley Tng wrote: > Hi, > > Currently, guest_memfd doesn't update inode's i_blocks or i_bytes at > all. Hence, st_blocks in the struct populated by a userspace fstat() > call on a guest_memfd will always be 0. This patch series makes > guest_memfd track the amount of memory allocated on an inode, which > allows fstat() to accurately report that on requests from userspace. > > The inode's i_blocks and i_bytes fields are updated when the folio is > associated or disassociated from the guest_memfd inode, which are at > allocation and truncation times respectively. > > To update inode fields at truncation time, this series implements a > custom truncation function for guest_memfd. An alternative would be to > update truncate_inode_pages_range() to return the number of bytes > truncated or add/use some hook. > > Implementing a custom truncation function was chosen to provide > flexibility for handling truncations in future when guest_memfd > supports sources of pages other than the buddy allocator. This > approach of a custom truncation function also aligns with shmem, which > has a custom shmem_truncate_range(). Just wondered how shmem does it: it's through dquot_alloc_block_nodirty() / dquot_free_block_nodirty(). It's a shame we can't just use folio_free(). Could we maybe have a different callback (when the mapping is still guaranteed to be around) from where we could update i_blocks on the freeing path? -- Cheers, David