Re: remove the last set_fs() in common code, and remove it for x86 and powerpc v2

Matthew Wilcox
2020-09-01T17:42:34Z

On Tue, Sep 01, 2020 at 06:25:12PM +0100, Al Viro wrote:
> On Tue, Sep 01, 2020 at 07:13:00PM +0200, Christophe Leroy wrote:
>
> > 10.92% dd [kernel.kallsyms] [k] iov_iter_zero
>
> Interesting... Could you get an instruction-level profile inside iov_iter_zero(),
> along with the disassembly of that sucker?

Also, does [1] make any difference? Probably not since it's translating
O flags into IOCB flags instead of RWF flags into IOCB flags. I wonder
if there's a useful trick we can play here ... something like:

static inline int iocb_flags(struct file *file)
{
int res = 0;
if (likely(!file->f_flags & O_APPEND | O_DIRECT | O_DSYNC | __O_SYNC)) && !IS_SYNC(file->f_mapping->host))
return res;
if (file->f_flags & O_APPEND)
res |= IOCB_APPEND;
if (file->f_flags & O_DIRECT)
res |= IOCB_DIRECT;
if ((file->f_flags & O_DSYNC) || IS_SYNC(file->f_mapping->host))
res |= IOCB_DSYNC;
if (file->f_flags & __O_SYNC)
res |= IOCB_SYNC;
return res;
}

Can we do something like force O_DSYNC to be set if the inode IS_SYNC()
at the time of open? Or is setting the sync bit on the inode required
to affect currently-open files?

[1] https://lore.kernel.org/linux-fsdevel/95de7ce4-9254-39f1-304f-4455f66bf0f4@kernel.dk/