On 20.01.22 15:39, Matthew Wilcox wrote: > On Thu, Jan 20, 2022 at 03:15:37PM +0100, David Hildenbrand wrote: >> On 17.01.22 14:31, zhangliang (AG) wrote: >>> Sure, I will do that :) >> >> I'm polishing up / testing the patches and might send something out for discussion shortly. >> Just a note that on my branch was a version with a wrong condition that should have been fixed now. >> >> I am still thinking about PTE mapped THP. For these, we'll always >> have page_count() > 1, essentially corresponding to the number of still-mapped sub-pages. >> >> So if we end up with a R/O mapped part of a THP, we'll always have to COW and cannot reuse ever, >> although it's really just a single process mapping the THP via PTEs. >> >> One approach would be to scan the currently locked page table for entries mapping >> this same page. If page_count() corresponds to that value, we know that only we are >> mapping the THP and there are no additional references. That would be a special case >> if we find an anon THP in do_wp_page(). Hm. > > You're starting to optimise for some pretty weird cases at that point. So your claim is that read-only, PTE mapped pages are weird? How do you come to that conclusion? If we adjust the THP reuse logic to split on additional references (page_count() == 1) -- similarly as suggested by Linus to fix the CVE -- we're going to end up with exactly that more frequently. > Anon THP is always going to start out aligned (and can be moved by > mremap()). Arguably it should be broken up if it's moved so it can be > reformed into aligned THPs by khugepaged. Can you elaborate, I'm missing the point where something gets moved. I don't care about mremap() at all here. 1. You have a read-only, PTE mapped THP 2. Write fault on the THP 3. We PTE-map the THP because we run into a false positive in our COW logic to handle COW on PTE 4. Write fault on the PTE 5. We always have to COW each and every sub-page and can never reuse, because page_count() > 1 That's essentially what reuse_swap_page() tried to handle before. Eventually optimizing for this is certainly the next step, but I'd like to document which effect the removal of reuse_swap_page() will have to THP. -- Thanks, David / dhildenb
