On Thu, Jan 20, 2022 at 04:26:22PM +0100, David Hildenbrand wrote: > 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? Because normally anon THP pages are PMD mapped. That's rather the point of anon THPs. > 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. I don't understand. Are we talking past each other? As I understand the situation we're talking about here, a process has created a THP, done something to cause it to be partially mapped (or mapped in a misaligned way) in its own address space, then forked, and we're trying to figure out if it's safe to reuse it? I say that situation is rare enough that it's OK to always allocate an order-0 page and copy into it. > > 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.
