linux-kernel - Re: [PATCH v3] x86/e820: Fix handling of subpage regions when calculating nosave ranges

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Message-ID: <292f6f59-5657-474f-b521-ef1816a5daf2@qtmlabs.xyz>
Date: Mon, 7 Apr 2025 08:12:42 +0700
From: Myrrh Periwinkle <myrrhperiwinkle@...labs.xyz>
To: Ingo Molnar <mingo@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org,
 Roberto Ricci <io@...icci.it>
Subject: Re: [PATCH v3] x86/e820: Fix handling of subpage regions when
 calculating nosave ranges

On 4/7/25 07:53, Myrrh Periwinkle wrote:

> On 4/7/25 01:36, Ingo Molnar wrote:
>
>> * Ingo Molnar<mingo@...nel.org> wrote:
>>
>>> * Myrrh Periwinkle<myrrhperiwinkle@...labs.xyz> wrote:
>>>
>>>> The current implementation of e820__register_nosave_regions suffers 
>>>> from
>>>> multiple serious issues:
>>>>   - The end of last region is tracked by PFN, causing it to find holes
>>>>     that aren't there if two consecutive subpage regions are present
>>>>   - The nosave PFN ranges derived from holes are rounded out 
>>>> (instead of
>>>>     rounded in) which makes it inconsistent with how explicitly 
>>>> reserved
>>>>     regions are handled
>>>>
>>>> Fix this by:
>>>>   - Treating reserved regions as if they were holes, to ensure 
>>>> consistent
>>>>     handling (rounding out nosave PFN ranges is more correct as the
>>>>     kernel does not use partial pages)
>>>>   - Tracking the end of the last RAM region by address instead of 
>>>> pages
>>>>     to detect holes more precisely
>>>>
>>>> Cc:stable@...r.kernel.org
>>>> Fixes: e5540f875404 ("x86/boot/e820: Consolidate 'struct e820_entry 
>>>> *entry' local variable names")
>>> So why is this SHA1 indicated as the root cause? AFAICS that commit
>>> does nothing but cleanups, so it cannot cause such regressions.
>> BTW.:
>>
>>   A) "It was the first random commit that seemed related, sry"
>>   B) "It's a 15 years old bug, but I wanted to indicate a fresh, 
>> 8-year old bug to get this into -stable. Busted!"
>
> You got me :) How did you know that this is a 15 years old bug? 
> (although I didn't think the age of the bug a patch fixes would affect 
> its chances of getting to -stable)
>
> This specific revision was picked since it's the latest one that this 
> patch can be straightforwardly applied to (there is a (trivial) merge 
> conflict with -stable, though).
>
> Later, I managed to track the buggy logic back to 1c10070a55a3 ("i386: 
> do not restore reserved memory after hibernation"), which I believe is 
> the very first occurrence of this bug. If you prefer, I can send a v4 
> with a more correct Fixes: tag (or feel free to do so yourself when 
> applying this patch).

I did some more digging and it seems like the buggy logic actually 
appeared all the way back in e8eff5ac294e ("[PATCH] Make swsusp avoid 
memory holes and reserved memory regions on x86_64") back when x86_64 
was a separate port, which was copied later by the i386 port in the 
commit I mentioned above, which would make this a 19 year old bug 
instead of 15.

>
>> ... are perfectly fine answers in my book. :-)
>>
>> I'm glad about the fixes, I'm just curious how the Fixes tag came about.
>>
>> Thanks,
>>
>>     Ingo
>
> Regards,
>
> Myrrh
>