On Tue, Jan 05, 2021 at 09:22:51PM +0000, Nadav Amit wrote: > > On Jan 5, 2021, at 12:39 PM, Andrea Arcangeli <[email protected]> wrote: > > > > On Tue, Jan 05, 2021 at 07:26:43PM +0000, Nadav Amit wrote: > >>> On Jan 5, 2021, at 10:20 AM, Andrea Arcangeli <[email protected]> wrote: > >>> > >>> On Fri, Dec 25, 2020 at 01:25:29AM -0800, Nadav Amit wrote: > >>>> Fixes: 0f8975ec4db2 ("mm: soft-dirty bits for user memory changes > >>>> tracking") > >>> > >>> Targeting a backport down to 2013 when nothing could wrong in practice > >>> with page_mapcount sounds backwards and unnecessarily risky. > >>> > >>> In theory it was already broken and in theory > >>> 09854ba94c6aad7886996bfbee2530b3d8a7f4f4 is absolutely perfect and the > >>> previous code of 2013 is completely wrong, but in practice the code > >>> from 2013 worked perfectly until Aug 21 2020. > >> > >> Well… If you consider the bug that Will recently fixed [1], then soft-dirty > >> was broken (for a different, yet related reason) since 0758cd830494 > >> ("asm-generic/tlb: avoid potential double flush”). > >> > >> This is not to say that I argue that the patch should be backported to > >> 2013, > >> just to say that memory corruption bugs can be unnoticed. > >> > >> [1] > >> https://patchwork.kernel.org/project/linux-mm/patch/[email protected]/ > > > > Is this a fix or a cleanup? > > > > The above is precisely what I said earlier that tlb_gather had no > > reason to stay in clear_refs and it had to use inc_tlb_flush_pending > > as mprotect, but it's not a fix? Is it? I suggested it as a pure > > cleanup. So again no backport required. The commit says fix this but > > it means "clean this up". > > It is actually a fix. I think the commit log is not entirely correct and > should include: > > Fixes: 0758cd830494 ("asm-generic/tlb: avoid potential double flush”). > > Since 0758cd830494, calling tlb_finish_mmu() without any previous call to > pte_free_tlb() and friends does not flush the TLB. The soft-dirty bug > producer that I sent fails without this patch of Will.
Yes, it's a fix, but I didn't rush it for 5.10 because I don't think rushing this sort of thing does anybody any favours. I agree that the commit log should be updated; I mentioned this report in the cover letter: https://lore.kernel.org/linux-mm/CA+32v5zzFYJQ7eHfJP-2OHeR+6p5PZsX=rdjnu6vgf3hlo+...@mail.gmail.com/ demonstrating that somebody has independently stumbled over the missing TLB invalidation in userspace, but it's not as bad as the other issues we've been discussing in this thread. Will
