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([2a01:4b00:bd21:4f00:7cc6:d3ca:494:116c]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e3ebd48eesm33912134f8f.30.2026.10.29.04.34.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 29 Oct 2026 04:34:05 -0700 (PDT) Message-ID: <69f1290c-1650-4ce4-bc0f-e8a3c03a5d14@gmail.com> Date: Thu, 29 Oct 2026 11:34:07 +0000 X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC 0/4] fs: support dma-buf backed direct I/O To: Christoph Hellwig Cc: Anuj Gupta , djwong@kernel.org, cem@kernel.org, brauner@kernel.org, axboe@kernel.dk, linux-block@vger.kernel.org, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, joshi.k@samsung.com, anuj20.g@samsung.com, nj.shetty@samsung.com References: <20261016091919.971295-1-anuj1072538@gmail.com> <4649a20f-aba9-418e-987f-66f21ab24410@gmail.com> <20261028162900.GA22640@lst.de> Content-Language: en-US From: Pavel Begunkov In-Reply-To: <20261028162900.GA22640@lst.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10/28/26 16:29, Christoph Hellwig wrote: > On Sun, Oct 18, 2026 at 11:18:27PM +0100, Pavel Begunkov wrote: >> file_ref_t f_ref; >> /* --- cacheline 3 boundary (192 bytes) --- */ >> + >> + /* >> + * A tag representing the underlying subsystem for dma-buf I/O. >> + * See struct dma_buf_io_ctx::importer_tag. >> + */ >> + unsigned long f_dma_io_tag; >> } __randomize_layout > > Growing strut file into a new cacheline just for dmabufs is a no-go.. I didn't check last time but there is space to spare. Maybe I don't know about all production enabled options growing mutexes. struct file { spinlock_t f_lock __attribute__((__aligned__(4))); /* 0 4 */ fmode_t f_mode; /* 4 4 */ const struct file_operations * f_op; /* 8 8 */ struct address_space * f_mapping; /* 16 8 */ void * private_data; /* 24 8 */ struct inode * f_inode; /* 32 8 */ unsigned int f_flags; /* 40 4 */ unsigned int f_iocb_flags; /* 44 4 */ const struct cred * f_cred; /* 48 8 */ struct fown_struct * f_owner; /* 56 8 */ /* --- cacheline 1 boundary (64 bytes) --- */ union { const struct path f_path; /* 64 16 */ struct path __f_path; /* 64 16 */ }; /* 64 16 */ union { struct mutex f_pos_lock __attribute__((__aligned__(8))); /* 80 24 */ u64 f_pipe; /* 80 8 */ } __attribute__((__aligned__(8))); /* 80 24 */ loff_t f_pos; /* 104 8 */ void * f_security; /* 112 8 */ errseq_t f_wb_err; /* 120 4 */ errseq_t f_sb_err; /* 124 4 */ /* --- cacheline 2 boundary (128 bytes) --- */ struct hlist_head * f_ep; /* 128 8 */ union { struct callback_head f_task_work __attribute__((__aligned__(8))); /* 136 16 */ struct llist_node f_llist; /* 136 8 */ struct file_ra_state f_ra; /* 136 32 */ freeptr_t f_freeptr; /* 136 8 */ } __attribute__((__aligned__(8))); /* 136 32 */ file_ref_t f_ref; /* 168 8 */ /* size: 176, cachelines: 3, members: 19 */ /* forced alignments: 3 */ /* last cacheline: 48 bytes */ } __attribute__((__aligned__(8))); > Also note that a file can be placed on multiple block devices when > using RAID or just btrfs placements (not sure about f2fs). Filesystems should set it to a super block or something similar. It's "unsigned long" for that exact reason. Shouldn't be a problem for md either. -- Pavel Begunkov