On 24 Jan 2019, at 09.35, Jan Kara [off-list ref] wrote:

On Wed 23-01-19 19:27:12, Javier González wrote:

quoted

quoted

On 9 Jan 2019, at 16.30, Kanchan Joshi [off-list ref] wrote:

Exiting write-hints are exposed to user-mode. There is a possiblity
of conflict if kernel happens to use those. This patch introduces four
write-hints for exclusive kernel-mode use.

Signed-off-by: Kanchan Joshi <redacted>
---
include/linux/fs.h | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 811c777..e8548eb 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h

@@ -291,6 +291,11 @@ enum rw_hint {

	WRITE_LIFE_MEDIUM	= RWH_WRITE_LIFE_MEDIUM,
	WRITE_LIFE_LONG		= RWH_WRITE_LIFE_LONG,
	WRITE_LIFE_EXTREME	= RWH_WRITE_LIFE_EXTREME,
+/* below ones are meant for in-kernel use */
+	KERN_WRITE_LIFE_SHORT,
+	KERN_WRITE_LIFE_MEDIUM,
+	KERN_WRITE_LIFE_LONG,
+	KERN_WRITE_LIFE_EXTREME
};

I think Jens and Dave meant kernel hints to go top down. This would also
give space for supporting more hints / streams from both ends for user
and kernel.

Yes, that was the idea however if I understand it right, the write hints do
not really have to be consistent boot-to-boot since they aren't stored
persistently by the disk, are they? If that's the case, it doesn't really
matter which numbers we pick.

I guess this is implementation specific. Some times the drive will want
to store this to improve GC. For the current "coldness" hint I does not
matter much, but if the hint were to express other metric it can become
relevant.

Anyway, the comment was more to separate user / kernel hints and allow
them to grow from the ends.

One thing I don't quite like is the naming of KERN_WRITE_LIFE_SHORT etc.. It
is upto filesystem to assign meanings to the write hints. So I think it is
enough to provide something like KERN_WRITE_HINT_MIN which is the first
hint available to the kernel and then the number of hints available to the
kernel.

Makes sense to me. Then we can rename the hint for each FS to give it
proper mening.

								Honza

--
Jan Kara [off-list ref]
SUSE Labs, CR

Javier