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Shutemov" , Jerome Glisse , Matthew Wilcox , Andrew Morton , David Hildenbrand , Andrea Arcangeli Subject: Re: [PATCH v6 06/23] mm/shmem: Handle uffd-wp special pte in page fault handler Message-ID: References: <20211115075522.73795-1-peterx@redhat.com> <20211115075522.73795-7-peterx@redhat.com> <6587740.tPqSsf18xI@nvdebian> MIME-Version: 1.0 In-Reply-To: <6587740.tPqSsf18xI@nvdebian> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 2D17B8000B X-Stat-Signature: b6tk8gnknn893n1k1zatbocf3egxu57d Authentication-Results: imf02.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=Vv8P+x0I; spf=none (imf02.hostedemail.com: domain of peterx@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=peterx@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-HE-Tag: 1639635460-626635 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 Thu, Dec 16, 2021 at 04:56:42PM +1100, Alistair Popple wrote: > On Monday, 15 November 2021 6:55:05 PM AEDT Peter Xu wrote: > > [...] > > > diff --git a/mm/memory.c b/mm/memory.c > > index d5966d9e24c3..e8557d43a87d 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -3452,6 +3452,43 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf) > > return 0; > > } > > > > +static vm_fault_t pte_marker_clear(struct vm_fault *vmf) > > +{ > > + vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd, > > + vmf->address, &vmf->ptl); > > + /* > > + * Be careful so that we will only recover a special uffd-wp pte into a > > + * none pte. Otherwise it means the pte could have changed, so retry. > > + */ > > + if (is_pte_marker(*vmf->pte)) > > + pte_clear(vmf->vma->vm_mm, vmf->address, vmf->pte); > > + pte_unmap_unlock(vmf->pte, vmf->ptl); > > + return 0; > > +} > > + > > +/* > > + * This is actually a page-missing access, but with uffd-wp special pte > > + * installed. It means this pte was wr-protected before being unmapped. > > + */ > > +static vm_fault_t pte_marker_handle_uffd_wp(struct vm_fault *vmf) > > +{ > > + /* Careful! vmf->pte unmapped after return */ > > + if (!pte_unmap_same(vmf)) > > Hasn't vmf->pte already been unmapped by do_swap_page() by the time we get > here? Great catch, thanks! It was needed before with the "swap special pte" version because that was handled outside do_swap_page(). After the rebase I forgot to remove it. I believe it didn't crash simply because we've got commit 2ca99358671a ("mm: clear vmf->pte after pte_unmap_same() returns", 2021-11-06) very recently so it just became a safe no-op, so all things will still work. I'll drop it. > > > + return 0; > > + > > + /* > > + * Just in case there're leftover special ptes even after the region > > + * got unregistered - we can simply clear them. We can also do that > > + * proactively when e.g. when we do UFFDIO_UNREGISTER upon some uffd-wp > > + * ranges, but it should be more efficient to be done lazily here. > > + */ > > + if (unlikely(!userfaultfd_wp(vmf->vma) || vma_is_anonymous(vmf->vma))) > > + return pte_marker_clear(vmf); > > + > > + /* do_fault() can handle pte markers too like none pte */ > > + return do_fault(vmf); > > +} > > + > > static vm_fault_t handle_pte_marker(struct vm_fault *vmf) > > { > > swp_entry_t entry = pte_to_swp_entry(vmf->orig_pte); > > @@ -3465,8 +3502,11 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf) > > if (WARN_ON_ONCE(vma_is_anonymous(vmf->vma) || !marker)) > > return VM_FAULT_SIGBUS; > > > > - /* TODO: handle pte markers */ > > - return 0; > > + if (marker & PTE_MARKER_UFFD_WP) > > Can we make this check `marker == PTE_MARKER_UFFD_WP`? There is currently only > one user of pte markers, and from what I can tell pte_marker_handle_uffd_wp() > wouldn't do the correct thing if other users were added because it could clear > non-uffd-wp markers. I don't think it's worth making it do the right thing now, > but a comment noting that would be helpful. Sure thing, and yeah I agree it's trivial and shouldn't matter in real-life. I'll change it to "marker == PTE_MARKER_UFFD_WP" as you suggested, so if there's surprise we'll get a sigbus. Thanks, > > > + return pte_marker_handle_uffd_wp(vmf); > > + > > + /* This is an unknown pte marker */ > > + return VM_FAULT_SIGBUS; > > } -- Peter Xu