Re: [RFC 0/4] fs: support dma-buf backed direct I/O

[Pavel Begunkov <asml.silence@gmail.com>]

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