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Wed, 27 Sep 2023 14:08:55 -0700 (PDT) MIME-Version: 1.0 References: <20230923013148.1390521-1-surenb@google.com> <20230923013148.1390521-3-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Wed, 27 Sep 2023 14:08:42 -0700 Message-ID: Subject: Re: [PATCH v2 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: B5BCE180008 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: hnridnrsu6mu6a6cnd7pezshktb93w57 X-HE-Tag: 1695848936-283042 X-HE-Meta: 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 1Hg3gw10 zZqd3o09L6usvsYYccJKQd1RuXiedDytyTkjSYW3izFT18i3hj5jNGbdF9DDU4fIlanNjaT/qrbPdS4Gd3FVEJ7bj+XGZlMdQpgHKCQz1frqKzKnLsdPO2C0dT1W04wUwRURwh4GjLNl9R6tDHDyVjGiBU/wVOewBCJS1RhkGsjUge1YdHZcP9X6gGE0NH97ZV83gCMiPU04jj0YDMm7DkfGSx63sXw6zrccuiWvTRSljz7GFIl7E3L8bXVvbEN1leAE1QxElSMzQ13cGEGWmyR3Tazix8MSKaRM4RERHGqL+vtwBAJ/7P+hJBhx796LE7tH2oHEE1jrJdZU= 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 Wed, Sep 27, 2023 at 1:42=E2=80=AFPM Suren Baghdasaryan wrote: > > On Wed, Sep 27, 2023 at 1:04=E2=80=AFPM Jann Horn wrot= e: > > > > On Wed, Sep 27, 2023 at 8:08=E2=80=AFPM Suren Baghdasaryan wrote: > > > On Wed, Sep 27, 2023 at 5:47=E2=80=AFAM Jann Horn = wrote: > > > > On Sat, Sep 23, 2023 at 3:31=E2=80=AFAM Suren Baghdasaryan wrote: > > > > > From: Andrea Arcangeli > > > > > > > > > > This implements the uABI of UFFDIO_REMAP. > > > > > > > > > > Notably one mode bitflag is also forwarded (and in turn known) by= the > > > > > lowlevel remap_pages method. > > > > > > > > > > Signed-off-by: Andrea Arcangeli > > > > > Signed-off-by: Suren Baghdasaryan > > [...] > > > > > + /* > > > > > + * folio_referenced walks the anon_vma ch= ain > > > > > + * without the folio lock. Serialize agai= nst it with > > > > > + * the anon_vma lock, the folio lock is n= ot enough. > > > > > + */ > > > > > + src_anon_vma =3D folio_get_anon_vma(src_f= olio); > > > > > + if (!src_anon_vma) { > > > > > + /* page was unmapped from under u= s */ > > > > > + 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; > > > > > + /* now we can block and wait */ > > > > > + anon_vma_lock_write(src_anon_vma)= ; > > > > > + goto retry; > > > > > + } > > > > > + } > > > > > > > > So at this point we have: > > > > > > > > - the current src_pte > > > > - some referenced+locked src_folio that used to be mapped exclusiv= ely > > > > at src_addr > > > > - (the anon_vma associated with the src_folio) > > > > > > > > > + err =3D remap_anon_pte(dst_mm, src_mm, dst_vma, = src_vma, > > > > > + dst_addr, src_addr, dst_pte,= src_pte, > > > > > + orig_dst_pte, orig_src_pte, > > > > > + dst_ptl, src_ptl, src_folio)= ; > > > > > > > > And then this will, without touching folio mapcounts/refcounts, del= ete > > > > the current PTE at src_addr, and create a PTE at dst_addr pointing = to > > > > the old src_folio, leading to incorrect refcounts/mapcounts? > > > > > > I assume this still points to the missing previous_src_pte check > > > discussed in the previous comments. Is that correct or is there yet > > > another issue? > > > > This is still referring to the missing previous_src_pte check. > > > > > > > > > > > + } else { > > > > [...] > > > > > + } > > > > > + > > > > > +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); > > > > > + mmu_notifier_invalidate_range_end(&range); > > > > > + > > > > > + return err; > > > > > +} > > > > [...] > > > > > +ssize_t remap_pages(struct mm_struct *dst_mm, struct mm_struct *= src_mm, > > > > > + unsigned long dst_start, unsigned long src_st= art, > > > > > + unsigned long len, __u64 mode) > > > > > +{ > > > > > + struct vm_area_struct *src_vma, *dst_vma; > > > > > + unsigned long src_addr, dst_addr; > > > > > + pmd_t *src_pmd, *dst_pmd; > > > > > + long err =3D -EINVAL; > > > > > + ssize_t moved =3D 0; > > > > > + > > > > > + /* > > > > > + * Sanitize the command parameters: > > > > > + */ > > > > > + BUG_ON(src_start & ~PAGE_MASK); > > > > > + BUG_ON(dst_start & ~PAGE_MASK); > > > > > + BUG_ON(len & ~PAGE_MASK); > > > > > + > > > > > + /* Does the address range wrap, or is the span zero-sized= ? */ > > > > > + BUG_ON(src_start + len <=3D src_start); > > > > > + BUG_ON(dst_start + len <=3D dst_start); > > > > > + > > > > > + /* > > > > > + * Because these are read sempahores there's no risk of l= ock > > > > > + * inversion. > > > > > + */ > > > > > + mmap_read_lock(dst_mm); > > > > > + if (dst_mm !=3D src_mm) > > > > > + mmap_read_lock(src_mm); > > > > > + > > > > > + /* > > > > > + * Make sure the vma is not shared, that the src and dst = remap > > > > > + * ranges are both valid and fully within a single existi= ng > > > > > + * vma. > > > > > + */ > > > > > + src_vma =3D find_vma(src_mm, src_start); > > > > > + if (!src_vma || (src_vma->vm_flags & VM_SHARED)) > > > > > + goto out; > > > > > + if (src_start < src_vma->vm_start || > > > > > + src_start + len > src_vma->vm_end) > > > > > + goto out; > > > > > + > > > > > + dst_vma =3D find_vma(dst_mm, dst_start); > > > > > + if (!dst_vma || (dst_vma->vm_flags & VM_SHARED)) > > > > > + goto out; > > > > > + if (dst_start < dst_vma->vm_start || > > > > > + dst_start + len > dst_vma->vm_end) > > > > > + goto out; > > > > > + > > > > > + err =3D validate_remap_areas(src_vma, dst_vma); > > > > > + if (err) > > > > > + goto out; > > > > > + > > > > > + for (src_addr =3D src_start, dst_addr =3D dst_start; > > > > > + src_addr < src_start + len;) { > > > > > + spinlock_t *ptl; > > > > > + pmd_t dst_pmdval; > > > > > + unsigned long step_size; > > > > > + > > > > > + BUG_ON(dst_addr >=3D dst_start + len); > > > > > + /* > > > > > + * Below works because anonymous area would not h= ave a > > > > > + * transparent huge PUD. If file-backed support i= s added, > > > > > + * that case would need to be handled here. > > > > > + */ > > > > > + src_pmd =3D mm_find_pmd(src_mm, src_addr); > > > > > + if (unlikely(!src_pmd)) { > > > > > + if (!(mode & UFFDIO_REMAP_MODE_ALLOW_SRC_= HOLES)) { > > > > > + err =3D -ENOENT; > > > > > + break; > > > > > + } > > > > > + src_pmd =3D mm_alloc_pmd(src_mm, src_addr= ); > > > > > + if (unlikely(!src_pmd)) { > > > > > + err =3D -ENOMEM; > > > > > + break; > > > > > + } > > > > > + } > > > > > + dst_pmd =3D mm_alloc_pmd(dst_mm, dst_addr); > > > > > + if (unlikely(!dst_pmd)) { > > > > > + err =3D -ENOMEM; > > > > > + break; > > > > > + } > > > > > + > > > > > + dst_pmdval =3D pmdp_get_lockless(dst_pmd); > > > > > + /* > > > > > + * If the dst_pmd is mapped as THP don't override= it and just > > > > > + * be strict. If dst_pmd changes into TPH after t= his check, the > > > > > + * remap_pages_huge_pmd() will detect the change = and retry > > > > > + * while remap_pages_pte() will detect the change= and fail. > > > > > + */ > > > > > + if (unlikely(pmd_trans_huge(dst_pmdval))) { > > > > > + err =3D -EEXIST; > > > > > + break; > > > > > + } > > > > > + > > > > > + ptl =3D pmd_trans_huge_lock(src_pmd, src_vma); > > > > > + if (ptl && !pmd_trans_huge(*src_pmd)) { > > > > > + spin_unlock(ptl); > > > > > + ptl =3D NULL; > > > > > + } > > > > > > > > This still looks wrong - we do still have to split_huge_pmd() > > > > somewhere so that remap_pages_pte() works. > > > > > > Hmm, I guess this extra check is not even needed... > > > > Hm, and instead we'd bail at the pte_offset_map_nolock() in > > remap_pages_pte()? I guess that's unusual but works... > > Yes, that's what I was thinking but I agree, that seems fragile. Maybe > just bail out early if (ptl && !pmd_trans_huge())? No, actually we can still handle is_swap_pmd() case by splitting it and remapping the individual ptes. So, I can bail out only in case of pmd_devmap(). > > > > > (It would be a thing to look out for if anyone tried to backport this, > > since the checks in pte_offset_map_nolock() were only introduced in > > 6.5, but idk if anyone's doing that)