> From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Sent: Monday, February 04, 2019 7:28 PM
> On Thu, 31 Jan 2019, Huaisheng Ye wrote:
>
> > From: Huaisheng Ye <yehs1@xxxxxxxxxx>
> >
> > This patch set could be used for dm-writecache when use persistent
> > memory as cache data device.
> >
> > Patch 1 and 2 go towards removing unused parameter and codes which
> > actually doesn't really work.
>
> I agree that there is some unused variables and code, but I would let it
> be as it is. The processors can write data to persistent memory either by
> using non-temporal stores (the movnti instruction) or by normal stores
> followed by the clwb instruction.
>
> Currently, the movnti instruction is faster - however, it may be possible
> that with some newer processors, the clwb instruction could be faster -
> and in that case, we need the code that you have removed.
>
> I would like to keep both flush strategies around (movnti and clwb), so
> that we can test how do they perform on various processors.
> Unfortunatelly, some upstream developers hate code with #ifdefs :-(
>
> Note that compiler optimizations already remove the unused parameter and
> the impossible code behind "if (WC_MODE_PMEM(wc)) if (!WC_MODE_PMEM(wc))".
Hi Mikulas,
Thanks for your reply, now I could understand the code flow of dm-writecache better
than before.
In the process of playing around the code, I found that writecache_flush would try
to free earlier committed entry with lower seq-count. More seriously is that, writecache_flush
must check it for all entries which hasn't been committed. That's a lot of work to do when
there are many entries need to be flushed.
I have a plan for writecache_map to avoid using free entry when the committed writecache
has been hit. Does it make sense to simple the code flow, especially for saving additional rb-tree
node insertion and free steps?
Cheers,
Huaisheng Ye
> > Patch 3 and 4 are targeted at solving problem fn ctr failed to work
> > due to invalid magic or version, which is caused by the super block
> > of pmem has messy data stored.
>
> LVM zeros the beginning of new logical volumes, so there should be no
> problem with it. If the user wants to use the writecache target without
> LVM, he should zero the superblock with dd if=/dev/zero of=/dev/pmem0
> bs=4k count=1
>
> Note that other device mapper targets also follow this policy - for
> example see drivers/md/dm-snap-persistent.c:
> if (le32_to_cpu(dh->magic) == 0) {
> *new_snapshot = 1;
> return 0;
> }
>
> if (le32_to_cpu(dh->magic) != SNAP_MAGIC) {
> DMWARN("Invalid or corrupt snapshot");
> r = -ENXIO;
> goto bad;
> }
>
> So, I think there is no need for these patches - dm-writecache just does
> what others targets do.
>
> > Patch 5 is used for getting the status of seq_count.
>
> It may be accepted if other LVM team members find some use for this value.
>
> Mikulas
>
> > Changes Since v2:
> > - seq_count is important for flush operations, output it within status
> > for debugging and analyzing code behavior.
> > [1]: https://lkml.org/lkml/2019/1/3/43
> > [2]: https://lkml.org/lkml/2019/1/9/6
> >
> > Huaisheng Ye (5):
> > dm-writecache: remove unused size to writecache_flush_region
> > dm-writecache: get rid of memory_data flush to writecache_flush_entry
> > dm-writecache: expand pmem_reinit for struct dm_writecache
> > Documentation/device-mapper: add optional parameter reinit
> > dm-writecache: output seq_count within status
> >
> > Documentation/device-mapper/writecache.txt | 4 ++++
> > drivers/md/dm-writecache.c | 23 +++++++++++++----------
> > 2 files changed, 17 insertions(+), 10 deletions(-)
> >
> > --
> > 1.8.3.1
> >
> >
--
dm-devel mailing list
dm-devel@xxxxxxxxxx
https://www.redhat.com/mailman/listinfo/dm-devel
