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

[Ira Weiny]

On Tue, Feb 09, 2021 at 11:09:31AM -0800, Andrew Morton wrote:
> On Tue, 9 Feb 2021 16:11:23 +0100 David Sterba <dsterba@xxxxxxx> wrote:
> 
> > On Fri, Feb 05, 2021 at 03:23:00PM -0800, ira.weiny@xxxxxxxxx wrote:
> > > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> > > 
> > > 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.
> > 
> > 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.
> 
> It would be best to merge [1/4] via the btrfs tree.  Please add my
> 
> Acked-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> 
> 
> Although I think it would be better if [1/4] merely did the code
> movement.  Adding those BUG_ON()s is a semantic/functional change and
> really shouldn't be bound up with the other things this patch series
> does.

I proposed this too and was told 'no'...

<quote>
If we put in into a separate patch, someone will suggest backing out the
patch which tells us that there's a problem.
</quote>
	-- https://lore.kernel.org/lkml/20201209201415.GT7338@xxxxxxxxxxxxxxxxxxxx/

> This logically separate change raises questions such as
> 
> - What is the impact on overall code size?  Not huge, presumably, but
>   every little bit hurts.
> 
> - Additional runtime costs of those extra comparisons?
> 
> - These impacts could be lessened by using VM_BUG_ON() rather than
>   BUG_ON() - should we do this?

<sigh>  I lost that argument last time around.

<quote>
BUG() is our only option here.  Both limiting how much we copy or
copying the requested amount result in data corruption or leaking
information to a process that isn't supposed to see it.
</quote>

https://lore.kernel.org/lkml/20201209040312.GN7338@xxxxxxxxxxxxxxxxxxxx/

CC'ing Matthew because I _really_ don't want to argue this any longer.

> 
> - Linus reeeeeeeally doesn't like new BUG_ON()s.  Maybe you can sneak
>   it past him ;)

I'm worried too...  :-(

> 
> See what I mean?

Yes I do however ...  see above...  :-/

Ira

> I do think it would be best to take those assertions
> out of the patch and to propose them separately, at a later time.
>