linux-kernel - Re: [(resend) PATCH v3: {linux-4.14.y} ] dm cache: submit writethrough writes in parallel to origin and cache

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Date:   Wed, 5 Aug 2020 16:32:42 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     John Donnelly <john.p.donnelly@...cle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org
Subject: Re: [(resend) PATCH v3: {linux-4.14.y} ] dm cache: submit
 writethrough writes in parallel to origin and cache

On Tue, Aug 04, 2020 at 02:20:38PM -0400, Mike Snitzer wrote:
> On Tue, Aug 04 2020 at  8:47am -0400,
> Greg KH <gregkh@...uxfoundation.org> wrote:
> 
> > On Tue, Aug 04, 2020 at 07:33:05AM -0500, John Donnelly wrote:
> > > From: Mike Snitzer <snitzer@...hat.com>
> > > 
> > > Discontinue issuing writethrough write IO in series to the origin and
> > > then cache.
> > > 
> > > Use bio_clone_fast() to create a new origin clone bio that will be
> > > mapped to the origin device and then bio_chain() it to the bio that gets
> > > remapped to the cache device.  The origin clone bio does _not_ have a
> > > copy of the per_bio_data -- as such check_if_tick_bio_needed() will not
> > > be called.
> > > 
> > > The cache bio (parent bio) will not complete until the origin bio has
> > > completed -- this fulfills bio_clone_fast()'s requirements as well as
> > > the requirement to not complete the original IO until the write IO has
> > > completed to both the origin and cache device.
> > > 
> > > Signed-off-by: Mike Snitzer <snitzer@...hat.com>
> > > 
> > > (cherry picked from commit 2df3bae9a6543e90042291707b8db0cbfbae9ee9)
> > > 
> > > Fixes: 4ec34f2196d125ff781170ddc6c3058c08ec5e73 (dm bio record:
> > > save/restore bi_end_io and bi_integrity )
> > > 
> > > 4ec34f21 introduced a mkfs.ext4 hang on a LVM device that has been
> > > modified with lvconvert --cachemode=writethrough.
> > > 
> > > CC:stable@...r.kernel.org for 4.14.y
> > > 
> > > Signed-off-by: John Donnelly <john.p.donnelly@...cle.com>
> > > Reviewed-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@...cle.com>
> > > 
> > > conflicts:
> > > 	drivers/md/dm-cache-target.c. -  Corrected usage of
> > > 	writethrough_mode(&cache->feature) that was caught by
> > > 	compiler, and removed unused static functions : writethrough_endio(),
> > > 	defer_writethrough_bio(), wake_deferred_writethrough_worker()
> > > 	that generated warnings.
> > 
> > What is this "conflicts nonsense"?  You don't see that in any other
> > kernel patch changelog, do you?
> > 
> > > ---
> > > drivers/md/dm-cache-target.c | 92 ++++++++++++++++++--------------------------
> > > 1 file changed, 37 insertions(+), 55 deletions(-)
> > 
> > Please fix your email client up, it's totally broken and this does not
> > work at all and is getting frustrating from my side here.
> > 
> > Try sending emails to yourself and see if you can apply the patches, as
> > the one you sent here does not work, again:
> 
> John's inability to submit a patch that can apply aside: I do not like
> how this patch header is constructed (yet attributed "From" me).  It is
> devoid of detail as it relates to stable@.
> 
> Greg, please don't apply the v4 of this patch either.  I'll craft a
> proper stable@ patch that explains the reason for change and why we're
> left having to resolve conflicts in stable@.
> 
> But first I need to focus on sending DM changes to Linus for v5.9 merge.

Ok, no worries, I'll drop all of these from my review queue and wait for
something from you sometime in the future.

thanks,

greg k-h