On Thu, Jan 20, 2022 at 04:39:55PM +0100, David Hildenbrand wrote: > On 20.01.22 16:36, Matthew Wilcox wrote: > > 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. > > For example unless we are talking about *drumroll* COW handling. > > > > >> 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. > > 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()?
