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b=LnwmKIVnzsI7rLd7YC5Q5wJDjtFGkTHUPVgiP6UX4ocMfBcTmtiRhSmQ7cIXQ7R4x 4AYDB+T6Sxt95Wcu3I4kscolUBJrq/tS2AFtEmZsADVJgcq46iPQvArrp5yD9BN73i JDpIuT/Qo+Hf77HYVrklUc4ttOIjOWvTH5EVTYWNdD5wfoBuwTx8+ZUtMTZciMMYeK mxOF8SeVGzs0g3nKqCbBlIrq0sm3qh1MvTY9pHy27hZWoVz98CoemjsdvnPijsiplp 75Y5qb3WT5U6fmoefNJDo0Pe0vZhLDkj2KojQEvQ+gZfxC16hrAI/z/hj61xmbr+Wv 0fVASmfxEXPxA== Message-ID: <9ef9a0bd-4cff-4518-b7fb-e65c9b761a5a@kernel.org> Date: Tue, 24 Feb 2026 16:26:30 +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 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: 6tweh6m7x99sgpjdtduw96gkqfhsc9oa X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: CE9C62000D X-HE-Tag: 1771946799-541396 X-HE-Meta: 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 CkYXFXPD 69C6oqrDCKDbNhro= 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 2/24/26 00:42, Ackerley Tng wrote: > "David Hildenbrand (Arm)" writes: > >> 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(). > > Yup, Hugh pointed out that struct address_space *mapping (and inode) may already > have been freed by the time .free_folio() is called [1]. > > [1] https://lore.kernel.org/all/7c2677e1-daf7-3b49-0a04-1efdf451379a@google.com/ > >> 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? > > Do you mean that we should add a new callback to struct > address_space_operations? If that avoids having to implement truncation completely ourselves, that might be one option we could discuss, yes. Something like: diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst index 7c753148af88..94f8bb81f017 100644 --- a/Documentation/filesystems/vfs.rst +++ b/Documentation/filesystems/vfs.rst @@ -764,6 +764,7 @@ cache in your filesystem. The following members are defined: sector_t (*bmap)(struct address_space *, sector_t); void (*invalidate_folio) (struct folio *, size_t start, size_t len); bool (*release_folio)(struct folio *, gfp_t); + void (*remove_folio)(struct folio *folio); void (*free_folio)(struct folio *); ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter); int (*migrate_folio)(struct mapping *, struct folio *dst, @@ -922,6 +923,11 @@ cache in your filesystem. The following members are defined: its release_folio will need to ensure this. Possibly it can clear the uptodate flag if it cannot free private data yet. +``remove_folio`` + remove_folio is called just before the folio is removed from the + page cache in order to allow the cleanup of properties (e.g., + accounting) that needs the address_space mapping. + ``free_folio`` free_folio is called once the folio is no longer visible in the page cache in order to allow the cleanup of any private data. diff --git a/include/linux/fs.h b/include/linux/fs.h index 8b3dd145b25e..f7f6930977a1 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -422,6 +422,7 @@ struct address_space_operations { sector_t (*bmap)(struct address_space *, sector_t); void (*invalidate_folio) (struct folio *, size_t offset, size_t len); bool (*release_folio)(struct folio *, gfp_t); + void (*remove_folio)(struct folio *folio); void (*free_folio)(struct folio *folio); ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter); /* diff --git a/mm/filemap.c b/mm/filemap.c index 6cd7974d4ada..5a810eaacab2 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -250,8 +250,14 @@ void filemap_free_folio(struct address_space *mapping, struct folio *folio) void filemap_remove_folio(struct folio *folio) { struct address_space *mapping = folio->mapping; + void (*remove_folio)(struct folio *); BUG_ON(!folio_test_locked(folio)); + + remove_folio = mapping->a_ops->remove_folio; + if (unlikely(remove_folio)) + remove_folio(folio); + spin_lock(&mapping->host->i_lock); xa_lock_irq(&mapping->i_pages); __filemap_remove_folio(folio, NULL); Ideally we'd perform it under the lock just after clearing folio->mapping, but I guess that might be more controversial. For accounting you need the above might be good enough, but I am not sure for how many other use cases there might be. -- Cheers, David