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SubjectRe: block device/VM question
Date
"A month of sundays ago ptb wrote:"
> "A month of sundays ago Thunder from the hill wrote:"
> > > And for the O_DIRECT flag we seem to do alloc_kiovec(1, &f->f_iobuf).
> >
> > Perhaps we should go biovec here?

State of play:

1) I abandoned doing the dentry_open with O_DIRECT flags bit
from within the driver, because every external access then errored
with EINVAL without getting to my driver functions.

2) I then just tried opening the device normally from userspace
but with O_DIRECT set. This is harder than it should have been - I had
to set nearly everything in features.h on before I got the desired
compilation (was it _XOPEN_SOURCE that did it? I forget), and surprise,
surprise, I get exactly the same effects as in (1). Every access
errors with EINVAL and my driver code never gets called. An open
with code compiled without O_DIRECT works fine, as usual.

I put a little debugging in the driver open call. With O_DIRECT
in the userspace open:

#881[12]: _open have O_DIRECT iobuf 0xcf935000 for part 0
#883[12]: _open have i_mapping 0xcce7f874 for part 0
#885[12]: _open have a_ops 0xc029ef40 for part 0
#889[12]: _open have direct_IO 0xc013edc0 for part 0

(I presume the latter is generic_file_direct_IO - it's not in my
map, but it's static so shouldn't be).

and without:

#881[14]: _open have ~O_DIRECT iobuf 0x0 for part 0
#883[14]: _open have i_mapping 0xcce7f874 for part 0
#885[14]: _open have a_ops 0xc029ef40 for part 0
#889[14]: _open have direct_IO 0xc013edc0 for part 0

So it looks as though some driver cooperation is required in
order for direct_IO/O_DIRECT to work.

What? Anyone know?


Peter
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