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Fri, 15 Sep 2023 16:39:32 -0700 (PDT) MIME-Version: 1.0 References: <20230914152620.2743033-1-surenb@google.com> <20230914152620.2743033-3-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Fri, 15 Sep 2023 16:39:18 -0700 Message-ID: Subject: Re: [PATCH 2/3] userfaultfd: UFFDIO_REMAP uABI To: Jann Horn Cc: akpm@linux-foundation.org, viro@zeniv.linux.org.uk, brauner@kernel.org, shuah@kernel.org, aarcange@redhat.com, lokeshgidra@google.com, peterx@redhat.com, david@redhat.com, hughd@google.com, mhocko@suse.com, axelrasmussen@google.com, rppt@kernel.org, willy@infradead.org, Liam.Howlett@oracle.com, zhangpeng362@huawei.com, bgeffon@google.com, kaleshsingh@google.com, ngeoffray@google.com, jdduke@google.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 07C7680015 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: bbuntf5bez7o1ky55crjbaaw9xxz3pbw X-HE-Tag: 1694821173-871549 X-HE-Meta: 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 gTcqdQnj XKC7t3i6rrUgMnV6/BWDQriddrIDzd7QwvUFbm8iC2IqM9bGaeUDD85zXVHrKcUOlMiw3skdyXa/5m/hbVeSuYfGocBVNSZsJ2WidcdcszDZ80UhjQnf8UNT5NOpFkwkE52EGWfjsZJ5/lZLc5z63yDVPeLf4hLi92u8cZInc0Kl01IsBl01URggR2c5x53N07JGlx5PZCahz/srUOVl1ove8jqfZI1bEciB+gfZGM9wBQdNNHMo70fioRp1/QKjzAT0TswJRLatLXx/Kn/5bU0/A4zGY0icckXIiXQG8YOGlSwQ= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Sep 15, 2023 at 4:34=E2=80=AFPM Jann Horn wrote: > > On Thu, Sep 14, 2023 at 5:26=E2=80=AFPM Suren Baghdasaryan wrote: > > +/* > > + * The mmap_lock for reading is held by the caller. Just move the page > > + * from src_pmd to dst_pmd if possible, and return true if succeeded > > + * in moving the page. > > + */ > > +static int remap_pages_pte(struct mm_struct *dst_mm, > > + struct mm_struct *src_mm, > > + pmd_t *dst_pmd, > > + pmd_t *src_pmd, > > + struct vm_area_struct *dst_vma, > > + struct vm_area_struct *src_vma, > > + unsigned long dst_addr, > > + unsigned long src_addr, > > + __u64 mode) > > +{ > > + swp_entry_t entry; > > + pte_t orig_src_pte, orig_dst_pte; > > + spinlock_t *src_ptl, *dst_ptl; > > + pte_t *src_pte =3D NULL; > > + pte_t *dst_pte =3D NULL; > > + > > + struct folio *src_folio =3D NULL; > > + struct anon_vma *src_anon_vma =3D NULL; > > + struct anon_vma *dst_anon_vma; > > + struct mmu_notifier_range range; > > + int err =3D 0; > > + > > +retry: > > + dst_pte =3D pte_offset_map_nolock(dst_mm, dst_pmd, dst_addr, &d= st_ptl); > > + > > + /* If an huge pmd materialized from under us fail */ > > + if (unlikely(!dst_pte)) { > > + err =3D -EFAULT; > > + goto out; > > + } > > + > > + src_pte =3D pte_offset_map_nolock(src_mm, src_pmd, src_addr, &s= rc_ptl); > > + > > + /* > > + * We held the mmap_lock for reading so MADV_DONTNEED > > + * can zap transparent huge pages under us, or the > > + * transparent huge page fault can establish new > > + * transparent huge pages under us. > > + */ > > + if (unlikely(!src_pte)) { > > + err =3D -EFAULT; > > + goto out; > > + } > > + > > + BUG_ON(pmd_none(*dst_pmd)); > > + BUG_ON(pmd_none(*src_pmd)); > > + BUG_ON(pmd_trans_huge(*dst_pmd)); > > + BUG_ON(pmd_trans_huge(*src_pmd)); > > + > > + spin_lock(dst_ptl); > > + orig_dst_pte =3D *dst_pte; > > + spin_unlock(dst_ptl); > > + if (!pte_none(orig_dst_pte)) { > > + err =3D -EEXIST; > > + goto out; > > + } > > + > > + spin_lock(src_ptl); > > + orig_src_pte =3D *src_pte; > > + spin_unlock(src_ptl); > > + if (pte_none(orig_src_pte)) { > > + if (!(mode & UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES)) > > + err =3D -ENOENT; > > + else /* nothing to do to remap a hole */ > > + err =3D 0; > > + goto out; > > + } > > + > > + if (pte_present(orig_src_pte)) { > > + if (!src_folio) { > > + struct folio *folio; > > + > > + /* > > + * Pin the page while holding the lock to be su= re the > > + * page isn't freed under us > > + */ > > + spin_lock(src_ptl); > > + if (!pte_same(orig_src_pte, *src_pte)) { > > + spin_unlock(src_ptl); > > + err =3D -EAGAIN; > > + goto out; > > + } > > + > > + folio =3D vm_normal_folio(src_vma, src_addr, or= ig_src_pte); > > + if (!folio || !folio_test_anon(folio) || > > + folio_estimated_sharers(folio) !=3D 1) { > > + spin_unlock(src_ptl); > > + err =3D -EBUSY; > > + goto out; > > + } > > + > > + src_folio =3D folio; > > + folio_get(src_folio); > > + spin_unlock(src_ptl); > > + > > + /* try to block all concurrent rmap walks */ > > + if (!folio_trylock(src_folio)) { > > + pte_unmap(&orig_src_pte); > > + pte_unmap(&orig_dst_pte); > > + src_pte =3D dst_pte =3D NULL; > > + folio_lock(src_folio); > > + goto retry; > > + } > > + } > > + > > + if (!src_anon_vma) { > > + /* > > + * folio_referenced walks the anon_vma chain > > + * without the folio lock. Serialize against it= with > > + * the anon_vma lock, the folio lock is not eno= ugh. > > + */ > > + src_anon_vma =3D folio_get_anon_vma(src_folio); > > + if (!src_anon_vma) { > > + /* page was unmapped from under us */ > > + err =3D -EAGAIN; > > + goto out; > > + } > > + if (!anon_vma_trylock_write(src_anon_vma)) { > > + pte_unmap(&orig_src_pte); > > + pte_unmap(&orig_dst_pte); > > + src_pte =3D dst_pte =3D NULL; > > + anon_vma_lock_write(src_anon_vma); > > + goto retry; > > + } > > + } > > + > > + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, sr= c_mm, > > + src_addr, src_addr + PAGE_SIZE)= ; > > + mmu_notifier_invalidate_range_start_nonblock(&range); > > I'm pretty sure you're not allowed to use > mmu_notifier_invalidate_range_start_nonblock(). Use > mmu_notifier_invalidate_range_start() at a point where it's fine to > block; perhaps all the way up in remap_pages() around the whole loop > or something like that. mmu_notifier_invalidate_range_start_nonblock() > is special magic sauce for OOM reaping (with deceptively inviting > naming and no documentation that explains that this is evil hazmat > code), and if you chase down what various users of MMU notifiers do > when you just hand them a random notification where > mmu_notifier_range_blockable() is false, you end up in fun paths like > in KVM's kvm_mmu_notifier_invalidate_range_start(), which calls > gfn_to_pfn_cache_invalidate_start(), which does this: > > /* > * If the OOM reaper is active, then all vCPUs should have > * been stopped already, so perform the request without > * KVM_REQUEST_WAIT and be sad if any needed to be IPI'd. > */ > if (!may_block) > req &=3D ~KVM_REQUEST_WAIT; > > called =3D kvm_make_vcpus_request_mask(kvm, req, vcpu_bitmap); > > WARN_ON_ONCE(called && !may_block); > > Which means if you hit this codepath from a place like userfaultfd > where the process is probably still running (and not being OOM reaped > in a context where it's guaranteed to have been killed and stopped), > you could probably get KVM into a situation where it needs to wait for > vCPUs to acknowledge that they've processed a message before it's safe > to continue, but it can't wait because we're in nonsleepable context, > and then it just continues without waiting and presumably the KVM TLB > gets out of sync or something? Yeah, I was sceptical about this function too but it was very tempting :) Thanks for the warning, I'll move mmu_notifier_invalidate_range_start() to before we map the ptes because that's where we start the RCU read section. > > > > > + > > + double_pt_lock(dst_ptl, src_ptl); > > + > > + if (!pte_same(*src_pte, orig_src_pte) || > > + !pte_same(*dst_pte, orig_dst_pte) || > > + folio_estimated_sharers(src_folio) !=3D 1) { > > + double_pt_unlock(dst_ptl, src_ptl); > > + err =3D -EAGAIN; > > + goto out; > > + } > > + > > + BUG_ON(!folio_test_anon(src_folio)); > > + /* the PT lock is enough to keep the page pinned now */ > > + folio_put(src_folio); > > + > > + dst_anon_vma =3D (void *) dst_vma->anon_vma + PAGE_MAPP= ING_ANON; > > + WRITE_ONCE(src_folio->mapping, > > + (struct address_space *) dst_anon_vma); > > + WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, > > + dst_addr)= ); > > + > > + if (!pte_same(ptep_clear_flush(src_vma, src_addr, src_p= te), > > + orig_src_pte)) > > + BUG_ON(1); > > + > > + orig_dst_pte =3D mk_pte(&src_folio->page, dst_vma->vm_p= age_prot); > > + orig_dst_pte =3D maybe_mkwrite(pte_mkdirty(orig_dst_pte= ), > > + dst_vma); > > + > > + set_pte_at(dst_mm, dst_addr, dst_pte, orig_dst_pte); > > + > > + if (dst_mm !=3D src_mm) { > > + inc_mm_counter(dst_mm, MM_ANONPAGES); > > + dec_mm_counter(src_mm, MM_ANONPAGES); > > + } > > + > > + double_pt_unlock(dst_ptl, src_ptl); > > + > > + anon_vma_unlock_write(src_anon_vma); > > + mmu_notifier_invalidate_range_end(&range); > > + put_anon_vma(src_anon_vma); > > + src_anon_vma =3D NULL; > > + > > + /* unblock rmap walks */ > > + folio_unlock(src_folio); > > + src_folio =3D NULL; > > + > > + } else { > > + struct swap_info_struct *si; > > + int swap_count; > > + > > + entry =3D pte_to_swp_entry(orig_src_pte); > > + if (non_swap_entry(entry)) { > > + if (is_migration_entry(entry)) { > > + pte_unmap(&orig_src_pte); > > + pte_unmap(&orig_dst_pte); > > + src_pte =3D dst_pte =3D NULL; > > + migration_entry_wait(src_mm, src_pmd, > > + src_addr); > > + err =3D -EAGAIN; > > + } else > > + err =3D -EFAULT; > > + goto out; > > + } > > + > > + /* > > + * COUNT_CONTINUE to be returned is fine here, no need > > + * of follow all swap continuation to check against > > + * number 1. > > + */ > > + si =3D get_swap_device(entry); > > + if (!si) { > > + err =3D -EBUSY; > > + goto out; > > + } > > + > > + swap_count =3D swap_swapcount(si, entry); > > + put_swap_device(si); > > + if (swap_count !=3D 1) { > > + err =3D -EBUSY; > > + goto out; > > + } > > + > > + double_pt_lock(dst_ptl, src_ptl); > > + > > + if (!pte_same(*src_pte, orig_src_pte) || > > + !pte_same(*dst_pte, orig_dst_pte) || > > + swp_swapcount(entry) !=3D 1) { > > + double_pt_unlock(dst_ptl, src_ptl); > > + err =3D -EAGAIN; > > + goto out; > > + } > > + > > + if (pte_val(ptep_get_and_clear(src_mm, src_addr, src_pt= e)) !=3D > > + pte_val(orig_src_pte)) > > + BUG_ON(1); > > + set_pte_at(dst_mm, dst_addr, dst_pte, orig_src_pte); > > + > > + if (dst_mm !=3D src_mm) { > > + inc_mm_counter(dst_mm, MM_ANONPAGES); > > + dec_mm_counter(src_mm, MM_ANONPAGES); > > + } > > + > > + double_pt_unlock(dst_ptl, src_ptl); > > + } > > + > > +out: > > + if (src_anon_vma) { > > + anon_vma_unlock_write(src_anon_vma); > > + put_anon_vma(src_anon_vma); > > + } > > + if (src_folio) { > > + folio_unlock(src_folio); > > + folio_put(src_folio); > > + } > > + if (dst_pte) > > + pte_unmap(dst_pte); > > + if (src_pte) > > + pte_unmap(src_pte); > > + > > + return err; > > +}