On Thu, Dec 03, 2020 at 09:59:50PM -0800, Hugh Dickins wrote: > On Thu, 3 Dec 2020, Andrea Arcangeli wrote: > > On Thu, Dec 03, 2020 at 09:30:51PM -0500, Peter Xu wrote: > > > I'm just afraid there's no space left for a migration entry, because migration > > > entries fills in the pfn information into swp offset field rather than a real > > > offset (please refer to make_migration_entry())? I assume PFN can use any bit. > > > Or did I miss anything? > > > > > > I went back to see the original proposal from Hugh: > > > > > > IIUC you only need a single value, no need to carve out another whole > > > swp_type: could probably be swp_offset 0 of any swp_type other than 0. > > > > > > Hugh/Andrea, sorry if this is a stupid swap question: could you help explain > > > why swp_offset=0 won't be used by any swap device? I believe it's correct, > > > it's just that I failed to figure out the reason myself. :( > > > > > It's because swp_offset 0 is the offset of the swap header, and if we > ever used that when allocating swap, then the swap header would get > overwritten, and that swap area become unrecognizable next time. > > But I said it would be usable for UFFD with any swp_type other than 0, > because a swap entry of type 0, offset 0 is simply 0, which looks just > like no swap entry at all, and there are (or were: I might not be > up-to-date) benign races where a swap entry might get passed down but > then found to be 0, and that was understandable and permitted (yes, > I still see the "if (!entry.val) goto out;" in __swap_info_get()). > > And that might be related to pte_none() being 0 on most architectures > (not s390 IIRC): we need to distinguish none from swap. Though that > all gets complicated by the way the swp_entry is munged before being > put into a pte, and the x86 swap munging got more complicated when > L1TF was revealed (and accompanied by prot none munging too) - > search git log of v4.19 for x86/speculation/l1tf if you need to. My thanks to both of you for explaining the details. > > > > > Hugh may want to review if I got it wrong, but there's basically three > > ways. > > > > swp_type would mean adding one more reserved value in addition of > > SWP_MIGRATION_READ and SWP_MIGRATION_WRITE (kind of increasing > > SWP_MIGRATION_NUM to 3). > > I'm not very keen on actually using any of the SWP_MIGRATION defines, > partly because in principle UFFD should not depend on CONFIG_MIGRATION, > partly because the uffd_wp entry would not behave anything like a > migration entry (whose pfn should always indicate a locked page). > > swp_offset 0 of swp_type 1 perhaps? > > > > > swp_offset = 0 works in combination of SWP_MIGRATION_WRITE and > > SWP_MIGRATION_READ if we enforce pfn 0 is never used by the kernel > > (I'd feel safer with pfn value -1UL truncated to the bits of the swp > > offset, since the swp_entry format is common code). > > > > The bit I was suggesting is just one more bit like _PAGE_SWP_UFFD_WP > > from the pte, one that cannot ever be set in any swp entry today. I > > assume it can't be _PAGE_SWP_UFFD_WP since that already can be set but > > you may want to verify it... > > I don't see why you would need another bit for this. > > The code that checks non-present non-none entries in page table, > for whether they are actually swap or migration entries or whatever, > would now also check for swp_offset 0 of swp_type 1 and go off to > the UFFD WP processing if so. > > I didn't pay much attention to below, it seemed over-complicated. > And I don't think Peter's PROT_NONE alternative was unworkable, > but would have to be more careful about pfn and L1TF than shown. > And I am more comfortable to focus on the swap-like direction, > than think in two directions at once - never my strength! Yes, I think both of them may work, but I'll follow your advise on using swap entries, assuming easier and cleaner than _PAGE_PROTNONE. For example, current pte_present() does make more sense to return false for such an uffd-wp reserved pte. Then I won't make _PAGE_PROTNONE even more complicated too. So I guess I'll start with type==1 && offset==0. (PS: I still think "swp_entry(0, _UFFD_SWP_UFFD_WP) && !vma_is_anonymous(vma)" could also be a good candidate comparing to "swp_entry(1, 0)" considering type==1 here is kind of randomly chosen from all the other numbers except 0; but maybe that's not extremely important - the major logic should be the same) Thanks! -- Peter Xu
