On 02.12.22 12:03, David Hildenbrand wrote:
On 01.12.22 23:30, Andrew Morton wrote:
On Thu, 1 Dec 2022 16:42:52 +0100 David Hildenbrand david@redhat.com wrote:
On 01.12.22 16:28, Peter Xu wrote:
I didn't reply here because I have already replied with the question in previous version with a few attempts. Quotting myself:
https://lore.kernel.org/all/Y3KgYeMTdTM0FN5W@x1n/
The thing is recovering the pte into its original form is the safest approach to me, so I think we need justification on why it's always safe to set the write bit.I've also got another longer email trying to explain why I think it's the other way round to be justfied, rather than justifying removal of the write bit for a read migration entry, here:
And I disagree for this patch that is supposed to fix this hunk:
@@ -243,11 +243,15 @@ static bool remove_migration_pte(struct page *page, struct vm_area_struct *vma, entry = pte_to_swp_entry(*pvmw.pte); if (is_write_migration_entry(entry)) pte = maybe_mkwrite(pte, vma);
else if (pte_swp_uffd_wp(*pvmw.pte))pte = pte_mkuffd_wp(pte);if (pte_swp_uffd_wp(*pvmw.pte))pte = pte_mkuffd_wp(pte); } }David, I'm unclear on what you mean by the above. Can you please expand?
There is really nothing to justify the other way around here. If it's broken fix it independently and properly backport it independenty.
But we don't know about any such broken case.
I have no energy to spare to argue further ;)
This is a silent data loss bug, which is about as bad as it gets. Under obscure conditions, fortunately. But please let's keep working it. Let's aim for something minimal for backporting purposes. We can revisit any cleanliness issues later.
Okay, you activated my energy reserves.
David, do you feel that the proposed fix will at least address the bug without adverse side-effects?
Usually, when I suspect something is dodgy I unconsciously push back harder than I usually would.
I just looked into the issue once again and realized that this patch here (and also my alternative proposal) most likely tackles the more-generic issue from the wrong direction. I found yet another such bug (most probably two, just too lazy to write another reproducer). Migration code does the right thing here -- IMHO -- and the issue should be fixed differently.
I'm testing an alternative patch right now and will share it later today, along with a reproducer.
mprotect() reproducer attached.