[PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting

[Hugh Dickins]

On Sat, 26 Dec 2020, Hugh Dickins wrote:
> On Sun, 27 Dec 2020, Kirill A. Shutemov wrote:
> > 
> > Here's the fixup I have so far. It doesn't blow up immediately, but please
> > take a closer look. Who knows what stupid mistake I did this time. :/
> 
> It's been running fine on x86_64 for a couple of hours (but of course
> my testing is deficient, in not detecting the case Linus spotted).
> 
> But I just thought I'd try it on i386 (hadn't tried previous versions)
> and this has a new disappointment: crashes when booting, in the "check
> if the page fault is solved" in do_fault_around().  I imagine a highmem
> issue with kmap of the pte address, but I'm reporting now before looking
> into it further (but verified that current linux.git i386 boots up fine).

This patch (like its antecedents) moves the pte_unmap_unlock() from
after do_fault_around()'s "check if the page fault is solved" into
filemap_map_pages() itself (which apparently does not NULLify vmf->pte
after unmapping it, which is poor, but good for revealing this issue).
That looks cleaner, but of course there was a very good reason for its
original positioning.

Maybe you want to change the ->map_pages prototype, to pass down the
requested address too, so that it can report whether the requested
address was resolved or not.  Or it could be left to __do_fault(),
or even to a repeated fault; but those would be less efficient.

> 
> Maybe easily fixed: but does suggest this needs exposure in linux-next.
> 
> Hugh