On 2019-07-18, Rasmus Villemoes linux@rasmusvillemoes.dk wrote:
On 06/07/2019 16.57, Aleksa Sarai wrote:
--- a/fs/open.c +++ b/fs/open.c @@ -928,24 +928,32 @@ struct file *open_with_fake_path(const struct path *path, int flags, } EXPORT_SYMBOL(open_with_fake_path); -static inline int build_open_flags(int flags, umode_t mode, struct open_flags *op) +static inline int build_open_flags(struct open_how how, struct open_flags *op) {
How does passing such a huge struct by value affect code generation? Does gcc actually inline the function (and does it even inline the old one given that it's already non-trivial and has more than one caller).
I'm not sure, but I'll just do what you suggested with passing a const reference and just copying the few fields that actually are touched by this function.
diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h index 2868ae6c8fc1..e59917292213 100644 --- a/include/linux/fcntl.h +++ b/include/linux/fcntl.h @@ -4,13 +4,26 @@ #include <uapi/linux/fcntl.h> -/* list of all valid flags for the open/openat flags argument: */ +/* Should open_how.mode be set for older syscalls wrappers? */ +#define OPENHOW_MODE(flags, mode) \
- (((flags) | (O_CREAT | __O_TMPFILE)) ? (mode) : 0)
Typo: (((flags) & (O_CREAT | __O_TMPFILE)) ? (mode) : 0)
Yup, thanks. I'm not sure why my tests passed on v9 with this bug (they didn't pass in my v10-draft until I fixed this bug earlier today).
+/**
- Arguments for how openat2(2) should open the target path. If @extra is zero,
- then openat2(2) is identical to openat(2).
- @flags: O_* flags (unknown flags ignored).
- @mode: O_CREAT file mode (ignored otherwise).
should probably say "O_CREAT/O_TMPFILE file mode".
:+1:
- @upgrade_mask: restrict how the O_PATH may be re-opened (ignored otherwise).
- @resolve: RESOLVE_* flags (-EINVAL on unknown flags).
- @reserved: reserved for future extensions, must be zeroed.
- */
+struct open_how {
- __u32 flags;
- union {
__u16 mode;__u16 upgrade_mask;- };
- __u16 resolve;
So mode and upgrade_mask are naturally u16 aka mode_t. And yes, they probably never need to be used together, so the union works. That then makes the next member 2-byte aligned, so using a u16 for the resolve flags brings us to an 8-byte boundary, and 11 unused flag bits should be enough for a while. But it seems a bit artificial to cram all this together and then add 56 bytes of reserved space.
I will happily admit that padding to 64 bytes is probably _very_ extreme (I picked it purely because it's the size of a cache-line so anything bigger makes even less sense). I was hoping someone would suggest a better size once I posted the patchset, since I couldn't think of a good answer myself.
Do you have any suggestions for a better layout or padding size?