On Jan 5, 2021, at 10:20 AM, Andrea Arcangeli [off-list ref] wrote:

On Fri, Dec 25, 2020 at 01:25:29AM -0800, Nadav Amit wrote:

quoted

Fixes: 0f8975ec4db2 ("mm: soft-dirty bits for user memory changes tracking")

Targeting a backport down to 2013 when nothing could wrong in practice
with page_mapcount sounds backwards and unnecessarily risky.

In theory it was already broken and in theory
09854ba94c6aad7886996bfbee2530b3d8a7f4f4 is absolutely perfect and the
previous code of 2013 is completely wrong, but in practice the code
from 2013 worked perfectly until Aug 21 2020.

Well… If you consider the bug that Will recently fixed [1], then soft-dirty
was broken (for a different, yet related reason) since 0758cd830494
("asm-generic/tlb: avoid potential double flush”).

This is not to say that I argue that the patch should be backported to 2013,
just to say that memory corruption bugs can be unnoticed.

[1] https://patchwork.kernel.org/project/linux-mm/patch/20201210121110.10094-2-will@kernel.org/

Since nothing at all could go wrong in soft dirty and uffd-wp until
09854ba94c6aad7886996bfbee2530b3d8a7f4f4, the Fixes need to target
that, definitely not a patch from 2013.

This means the backports will apply clean, they don't need a simple
solution but one that doesn't regress the performance of open source
virtual machines and open source products using clear_refs and uffd-wp
in general.

To summarize my action items based your (and others) feedback on both
patches:

1. I will break the first patch into two different patches, one with the
“optimization” for write-unprotect, based on your feedback. It will not
be backported.

2. I will try to add a patch to avoid TLB flushes on
userfaultfd-writeunprotect. It will also not be backported.

3. Let me know if you want me to use your version of testing
mm_tlb_flush_pending() and conditionally flushing, wait for new version fro
you or Peter or to go with taking mmap_lock for write.

Thanks again,
Nadav