linux-kernel - Re: [PATCH hotfix 6.11] minmax: reduce egregious min/max macro expansion

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Message-ID: <cd6e01fb-4605-4f5f-835e-592ca70ebe03@stanley.mountain>
Date: Wed, 11 Sep 2024 20:25:33 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Hans de Goede <hdegoede@...hat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Richard Narron <richard@...zen.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Marcin Wojtas <marcin.s.wojtas@...il.com>,
	Russell King <linux@...linux.org.uk>,
	"David S . Miller" <davem@...emloft.net>,
	Arnd Bergmann <arnd@...nel.org>,
	Linus Torvalds <torvalds@...uxfoundation.org>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org, linux-staging@...ts.linux.dev,
	linux-mm@...ck.org, stable@...r.kernel.org
Subject: Re: [PATCH hotfix 6.11] minmax: reduce egregious min/max macro
 expansion

On Wed, Sep 11, 2024 at 06:24:54PM +0200, Hans de Goede wrote:
> Hi Lorenzo,
> 
> On 9/11/24 5:34 PM, Lorenzo Stoakes wrote:
> > Avoid nested min()/max() which results in egregious macro expansion.
> > 
> > This issue was introduced by commit 867046cc7027 ("minmax: relax check to
> > allow comparison between unsigned arguments and signed constants") [2].
> > 
> > Work has been done to address the issue of egregious min()/max() macro
> > expansion in commit 22f546873149 ("minmax: improve macro expansion and type
> > checking") and related, however it appears that some issues remain on more
> > tightly constrained systems.
> > 
> > Adjust a few known-bad cases of deeply nested macros to avoid doing so to
> > mitigate this. Porting the patch first proposed in [1] to Linus's tree.
> > 
> > Running an allmodconfig build using the methodology described in [2] we
> > observe a 35 MiB reduction in generated code.
> > 
> > The difference is much more significant prior to recent minmax fixes which
> > were not backported. As per [1] prior these the reduction is more like 200
> > MiB.
> > 
> > This resolves an issue with slackware 15.0 32-bit compilation as reported
> > by Richard Narron.
> > 
> > Presumably the min/max fixups would be difficult to backport, this patch
> > should be easier and fix's Richard's problem in 5.15.
> > 
> > [0]:https://lore.kernel.org/all/b97faef60ad24922b530241c5d7c933c@AcuMS.aculab.com/
> > [1]:https://lore.kernel.org/lkml/5882b96e-1287-4390-8174-3316d39038ef@lucifer.local/
> > [2]:https://lore.kernel.org/linux-mm/36aa2cad-1db1-4abf-8dd2-fb20484aabc3@lucifer.local/
> > 
> > Reported-by: Richard Narron <richard@...zen.com>
> > Closes: https://lore.kernel.org/all/4a5321bd-b1f-1832-f0c-cea8694dc5aa@aaazen.com/
> > Fixes: 867046cc7027 ("minmax: relax check to allow comparison between unsigned arguments and signed constants")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> 
> Thank you for your patch.
> 
> I must say that I'm not a fan of that this is patching 3 totally
> unrelated files here in a single patch.
> 
> This is e.g. going to be a problem if we need to revert one of
> the changes because of regressions...

These kinds of thing also complicates backporting to stable.  The stable kernel
developers like whole, unmodified patches.  So if we have to fix something in
sDIGIT_FITTING() then we'd want to pull this back instead of re-writing the fix
on top of the original define (unmodified patches).  But now we have to backport
the chunk which changes mvpp2 as well (whole patches).

regards,
dan carpenter