linux-kernel - Re: [PATCH 0/4] arch, mm: improve robustness of direct map manipulation

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Date:   Wed, 28 Oct 2020 12:17:35 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
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Subject: Re: [PATCH 0/4] arch, mm: improve robustness of direct map
 manipulation

On 28.10.20 12:09, Mike Rapoport wrote:
> On Tue, Oct 27, 2020 at 09:46:35AM +0100, David Hildenbrand wrote:
>> On 27.10.20 09:38, Mike Rapoport wrote:
>>> On Mon, Oct 26, 2020 at 06:05:30PM +0000, Edgecombe, Rick P wrote:
>>>
>>>> Beyond whatever you are seeing, for the latter case of new things
>>>> getting introduced to an interface with hidden dependencies... Another
>>>> edge case could be a new caller to set_memory_np() could result in
>>>> large NP pages. None of the callers today should cause this AFAICT, but
>>>> it's not great to rely on the callers to know these details.
> 
>>> A caller of set_memory_*() or set_direct_map_*() should expect a failure
>>> and be ready for that. So adding a WARN to safe_copy_page() is the first
>>> step in that direction :)
>>>
>>
>> I am probably missing something important, but why are we saving/restoring
>> the content of pages that were explicitly removed from the identity mapping
>> such that nobody will access them?
> 
> Actually, we should not be saving/restoring free pages during
> hibernation as there are several calls to mark_free_pages() that should
> exclude the free pages from the snapshot. I've tried to find why the fix
> that maps/unmaps a page to save it was required at the first place, but
> I could not find bug reports.
> 
> The closest I've got is an email from Rafael that asked to update
> "hibernate: handle DEBUG_PAGEALLOC" patch:
> 
> https://lore.kernel.org/linux-pm/200802200133.44098.rjw@sisk.pl/
> 
> Could it be that safe_copy_page() tries to workaround a non-existent
> problem?
> 

Clould be! Also see

https://lkml.kernel.org/r/38de5bb0-5559-d069-0ce0-daec66ef2746@suse.cz

which restores free page content based on more kernel parameters, not 
based on the original content.

-- 
Thanks,

David / dhildenb