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

From: Mike Rapoport <[email-masked]>
Date: Wed, 28 Oct 2020 13:09:45 +0200

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/[email-masked]/

Could it be that safe_copy_page() tries to workaround a non-existent
problem?

-- 
Sincerely yours,
Mike.