On 30.07.24 20:44, Peter Xu wrote:
On Mon, Jul 29, 2024 at 07:46:26PM +0200, David Hildenbrand wrote:
I see what you mean but this is a very similar pattern as used in
collapse_pte_mapped_thp(), no? There we have
start_pte = pte_offset_map_nolock(mm, pmd, haddr, &ptl);
...
if (!pml)
spin_lock(ptl);
...
pte_unmap(start_pte);
if (!pml)
spin_unlock(ptl);
Again, I don't have a strong opinion on this, but doing it more similar to
collapse_pte_mapped_thp() to obtain locks makes it clearer to me. But if I
am missing something obvious please shout and I'll change it.
Right.. I don't think that path can change the pte pgtable either, and
there is even the line Hugh left showing it's impossible:
if (!start_pte) /* mmap_lock + page lock should prevent this */
goto abort;
I was thinking maybe the page lock is the critical one, irrelevant of mmap
lock.
No strong opinion either. Not sure whether Hugh has some thoughts. But
maybe if we stick with pte_offset_map_nolock() and if there'll be a repost
anyway, we could add a similar comment like this one showing that the pte
pgtable should be actually as stable as the ptlock.
I think this all deserves some future cleanups :)
... for the time being I'll have to drop this patch completely. I should
have known that virt_to_page() does not work on kmap'ed pages, but part
of me didn't want to believe it.
CONFIG_HIGHPTE wants to make my life challenging :)
--
Cheers,
David / dhildenb
