>> Yes, we are talking past each other and no, I am talking about fully >> mapped THP, just mapped via PTEs. >> >> Please refer to our THP COW logic: do_huge_pmd_wp_page() > > You're going to have to be a bit more explicit. That's clearly handling > the case where there's a PMD mapping. If there is _also_ a PTE mapping, > then obviously the page is mapped more than once and can't be reused! > >>> >>>>> 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. >>> >>> I'm talking about step 0. How do we get a read-only, PTE-mapped THP? >>> Through mremap() or perhaps through an mprotect()/mmap()/munmap() that >>> failed to split the THP. >> >> do_huge_pmd_wp_page() > > I feel you could be a little more verbose about what you think is > going on here. Are you talking about the fallback: path where we > call __split_huge_pmd()? Sorry, I was less verbose because I was just sending out the patch+description to Linus' reply and was assuming you're going to read it anyways ;) Yes, I'm speaking about exactly that fallback path. -- Thanks, David / dhildenb
