On 13/12/2023 11:20, Jan Kara wrote:
To allow for further expansion, increase from unsigned int to unsigned
long.
Since the dma-buf driver prints the file->f_mode member, change the print
as necessary to deal with the larger size.
Signed-off-by: John Garry<john.g.garry@xxxxxxxxxx>
Uh, Al has more experience with fmode_t changes so I'd defer final decision
to him but to me this seems dangerous.
Ack
Firstly, this breaks packing of
struct file on 64-bit architectures and struct file is highly optimized for
cache efficiency (see the comment before the struct definition).
From pahole, I think that we still fit on the same 64B cacheline
(x86_64), but some padding has been added.
Before:
struct file {
union {
struct llist_node f_llist; /* 0 8 */
struct callback_head f_rcuhead
__attribute__((__aligned__(8))); /* 0 16 */
unsigned int f_iocb_flags; /* 0 4 */
} __attribute__((__aligned__(8))); /* 0 16 */
spinlock_t f_lock; /* 16 4 */
fmode_t f_mode; /* 20 4 */
atomic_long_t f_count; /* 24 8 */
struct mutex f_pos_lock; /* 32 32 */
/* --- cacheline 1 boundary (64 bytes) --- */
After:
struct file {
union {
struct llist_node f_llist /* 0 8 */
struct callback_head f_rcuhead
__attribute__((__aligned__(8))); /* 0 16 */
unsigned int f_iocb_flags; /* 0 4 */
} __attribute__((__aligned__(8))); /* 0 16 */
spinlock_t f_lock; /* 16 4 */
/* XXX 4 bytes hole, try to pack */
fmode_t f_mode; /* 24 8 */
atomic_long_t f_count; /* 32 8 */
struct mutex f_pos_lock; /* 40 32 */
/* --- cacheline 1 boundary (64 bytes) was 8 bytes ago --- */
Secondly
this will probably generate warnings on 32-bit architectures as there
sizeof(unsigned long) == sizeof(unsigned int) and so your new flags won't
fit anyway?
Right, it would then need to be unsigned long long. Or add another 32b
member for extended modes. There were no i386 build warnings.
Thanks,
John
