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Wed, 27 Sep 2023 13:04:43 -0700 (PDT) MIME-Version: 1.0 References: <20230923013148.1390521-1-surenb@google.com> <20230923013148.1390521-3-surenb@google.com> In-Reply-To: From: Jann Horn Date: Wed, 27 Sep 2023 22:04:06 +0200 Message-ID: Subject: Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI To: Suren Baghdasaryan 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: 4F443160013 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: o1kzcjkw8y5f8rmdmt9u9yqmxxzyxkmz X-HE-Tag: 1695845085-791939 X-HE-Meta: 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 0T8TDbQI EUgsw5CGXAG+0c8c00GgfvNKtxYPU4ZaGUMMYEaWoElUtJoVz/E18c0DXY4qg+NYWj65HvmIdyOhkh1+m7rz1+a2BCtnChZMaZ1/ssd0S2UM5al0f623MtpObwoaBBFWQWI1SPmAN1Yp9FH4C7KsULpKE5q3sG7HhRVVHEe7FwepA6gl932HQiFmXV15hfEEC5g84uvjl2fWwAI23D2tov3fr0EIjDfflIevNf2NsU1anWIj9QSLgHELe0KOFwulPs07EvMJCk4Djln9MlJk1aq0/LFag/gVV3RkDCmwNiJAyvkjVnAutCTF6moSmjU5IZNuT 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 8:08=E2=80=AFPM Suren Baghdasaryan wrote: > On Wed, Sep 27, 2023 at 5:47=E2=80=AFAM Jann Horn wrot= e: > > 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 chain > > > + * without the folio lock. Serialize against = it with > > > + * the anon_vma lock, the folio lock is not e= nough. > > > + */ > > > + 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; > > > + /* 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 exclusively > > 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, delete > > 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_start, > > > + 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 lock > > > + * 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 rema= p > > > + * ranges are both valid and fully within a single existing > > > + * 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 have = a > > > + * transparent huge PUD. If file-backed support is ad= ded, > > > + * 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_HOLE= S)) { > > > + 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 this = 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... (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)