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:   Tue, 4 Aug 2020 14:20:38 -0400
From:   Mike Snitzer <snitzer@...hat.com>
To:     Greg KH <gregkh@...uxfoundation.org>
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  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.

Thanks,
Mike