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Tsirkin" , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andy Lutomirski , Vlastimil Babka , Andrea Arcangeli , Sonny Rao , Minchan Kim , "Kirill A . Shutemov" , Dmitry Safonov Subject: Re: [PATCH v2 1/2] mm: Allow non-VM_DONTEXPAND and VM_PFNMAP mappings with MREMAP_DONTUNMAP Message-ID: <20210317204015.GM395976@xz-x1> References: <20210303175235.3308220-1-bgeffon@google.com> <20210317191334.564944-1-bgeffon@google.com> MIME-Version: 1.0 In-Reply-To: <20210317191334.564944-1-bgeffon@google.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Stat-Signature: upcmoqhmipa3x1if7urcjju3xseheg6k X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: AD03817CBE Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf20; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=216.205.24.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616013624-515120 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: Hi, Brian, On Wed, Mar 17, 2021 at 12:13:33PM -0700, Brian Geffon wrote: > Currently MREMAP_DONTUNMAP only accepts private anonymous mappings. This > change will widen the support to include any mappings which are not > VM_DONTEXPAND or VM_PFNMAP. The primary use case is to support > MREMAP_DONTUNMAP on mappings which may have been created from a memfd. > > This change which takes advantage of the existing check in vma_to_resize > for non-VM_DONTEXPAND and non-VM_PFNMAP mappings will cause > MREMAP_DONTUNMAP to return -EFAULT if such mappings are remapped. This > behavior is consistent with existing behavior when using mremap with > such mappings. > > Lokesh Gidra who works on the Android JVM, provided an explanation of how > such a feature will improve Android JVM garbage collection: > "Android is developing a new garbage collector (GC), based on userfaultfd. > The garbage collector will use userfaultfd (uffd) on the java heap during > compaction. On accessing any uncompacted page, the application threads will > find it missing, at which point the thread will create the compacted page > and then use UFFDIO_COPY ioctl to get it mapped and then resume execution. > Before starting this compaction, in a stop-the-world pause the heap will be > mremap(MREMAP_DONTUNMAP) so that the java heap is ready to receive > UFFD_EVENT_PAGEFAULT events after resuming execution. > > To speedup mremap operations, pagetable movement was optimized by moving > PUD entries instead of PTE entries [1]. It was necessary as mremap of even > modest sized memory ranges also took several milliseconds, and stopping the > application for that long isn't acceptable in response-time sensitive > cases. > > With UFFDIO_CONTINUE feature [2], it will be even more efficient to > implement this GC, particularly the 'non-moveable' portions of the heap. > It will also help in reducing the need to copy (UFFDIO_COPY) the pages. > However, for this to work, the java heap has to be on a 'shared' vma. > Currently MREMAP_DONTUNMAP only supports private anonymous mappings, this > patch will enable using UFFDIO_CONTINUE for the new userfaultfd-based heap > compaction." > > [1] https://lore.kernel.org/linux-mm/20201215030730.NC3CU98e4%25akpm@linux-foundation.org/ > [2] https://lore.kernel.org/linux-mm/20210302000133.272579-1-axelrasmussen@google.com/ > > Signed-off-by: Brian Geffon > --- > mm/mremap.c | 4 ---- > 1 file changed, 4 deletions(-) > > diff --git a/mm/mremap.c b/mm/mremap.c > index ec8f840399ed..2c57dc4bc8b6 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -653,10 +653,6 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr, > return ERR_PTR(-EINVAL); > } > > - if (flags & MREMAP_DONTUNMAP && (!vma_is_anonymous(vma) || > - vma->vm_flags & VM_SHARED)) > - return ERR_PTR(-EINVAL); > - > if (is_vm_hugetlb_page(vma)) > return ERR_PTR(-EINVAL); The code change seems to be not aligned with what the commit message said. Did you perhaps forget to add the checks against VM_DONTEXPAND | VM_PFNMAP? I'm guessing that (instead of commit message to be touched up) because you still attached the revert patch, then that check seems to be needed. Thanks, -- Peter Xu