linux-kernel - Re: [PATCH 0/4] btrfs: Convert kmaps to core page calls

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Date:   Tue, 9 Feb 2021 09:45:51 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     dsterba@...e.cz, Andrew Morton <akpm@...ux-foundation.org>,
        clm@...com, josef@...icpanda.com, dsterba@...e.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/4] btrfs: Convert kmaps to core page calls

On Tue, Feb 09, 2021 at 04:11:23PM +0100, David Sterba wrote:
> On Fri, Feb 05, 2021 at 03:23:00PM -0800, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > There are many places where kmap/<operation>/kunmap patterns occur.  We lift
> > these various patterns to core common functions and use them in the btrfs file
> > system.  At the same time we convert those core functions to use
> > kmap_local_page() which is more efficient in those calls.
> > 
> > I think this is best accepted through Andrew's tree as it has the mem*_page
> > functions in it.  But I'd like to get an ack from David or one of the other
> > btrfs maintainers before the btrfs patches go through.
> 
> I'd rather take the non-mm patches through my tree so it gets tested
> the same way as other btrfs changes, straightforward cleanups or not.

True.

> 
> This brings the question how to do that as the first patch should go
> through the MM tree. One option is to posptpone the actual cleanups
> after the 1st patch is merged but this could take a long delay.
> 
> I'd suggest to take the 1st patch within MM tree in the upcoming merge
> window and then I can prepare a separate pull with just the cleanups.
> Removing an inter-tree patch dependency was a sufficient reason for
> Linus in the past for such pull requests.

There are others how want this base patch too.[1]  So I like this option.

>
> > There are a lot more kmap->kmap_local_page() conversions but kmap_local_page()
> > requires some care with the unmapping order and so I'm still reviewing those
> > changes because btrfs uses a lot of loops for it's kmaps.
> 
> It sounds to me that converting the kmaps will take some time anyway so
> exporting the helpers first and then converting the subsystems might be
> a good option. In case you'd like to get rid of the simple cases in
> btrfs code now we can do the 2 pull requests.

I would really like to get the simple case out of the way because the next
series has more difficult changes and the simple cases always cause me trouble
when grepping/coccinelle'ing for things.

So I would like a follow on pull request if possible.  But I'm willing to do
what works best for you.

For now I will spin a new version with the changes you've requested ASAP.

Ira

[1] https://lore.kernel.org/linux-f2fs-devel/20210207190425.38107-1-chaitanya.kulkarni@wdc.com/