On Mon, Dec 21, 2020 at 4:00 PM Yu Zhao yuzhao@google.com wrote:
My first instinct is to be conservative and revert 09854ba94c6a ("mm: do_wp_page() simplification") so people are less likely to come back and complain about performance issues from holding mmap lock for write when clearing pte_write.
Well, the thing is, that simplificaiton was actually part of fixing a real regression wrt GUP.
Reverting that would break a308c71bf1e6 ("mm/gup: Remove enfornced COW mechanism").
And that one was the (better) fix for commit 17839856fd58 that fixed a real security issue, but did it with a big hammer that then caused problems for uffd-wp (and some other loads). There's a bit more context in the merge message in commit b25d1dc9474e Merge branch 'simplify-do_wp_page'.
So while that commit 09854ba94c6a on its own is "just" a simplification, it's actually part of a bigger series that fixes serious problems.
Linus