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[2003:cb:c733:6400:ae10:4bb7:9712:8548]) by smtp.gmail.com with ESMTPSA id k22-20020a7bc416000000b00404719b05b5sm11395985wmi.27.2023.10.09.07.38.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 09 Oct 2023 07:38:21 -0700 (PDT) Message-ID: <214b78ed-3842-5ba1-fa9c-9fa719fca129@redhat.com> Date: Mon, 9 Oct 2023 16:38:20 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI To: Suren Baghdasaryan , akpm@linux-foundation.org Cc: viro@zeniv.linux.org.uk, brauner@kernel.org, shuah@kernel.org, aarcange@redhat.com, lokeshgidra@google.com, peterx@redhat.com, hughd@google.com, mhocko@suse.com, axelrasmussen@google.com, rppt@kernel.org, willy@infradead.org, Liam.Howlett@oracle.com, jannh@google.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 References: <20231009064230.2952396-1-surenb@google.com> <20231009064230.2952396-3-surenb@google.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20231009064230.2952396-3-surenb@google.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 30606180013 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: hnhr9p64nbtuy89rorw3kx8w943pa4ki X-HE-Tag: 1696862310-393319 X-HE-Meta: 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 3ZbMPYxd 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 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 09.10.23 08:42, Suren Baghdasaryan wrote: > From: Andrea Arcangeli > > Implement the uABI of UFFDIO_MOVE ioctl. > UFFDIO_COPY performs ~20% better than UFFDIO_MOVE when the application > needs pages to be allocated [1]. However, with UFFDIO_MOVE, if pages are > available (in userspace) for recycling, as is usually the case in heap > compaction algorithms, then we can avoid the page allocation and memcpy > (done by UFFDIO_COPY). Also, since the pages are recycled in the > userspace, we avoid the need to release (via madvise) the pages back to > the kernel [2]. > We see over 40% reduction (on a Google pixel 6 device) in the compacting > thread’s completion time by using UFFDIO_MOVE vs. UFFDIO_COPY. This was > measured using a benchmark that emulates a heap compaction implementation > using userfaultfd (to allow concurrent accesses by application threads). > More details of the usecase are explained in [2]. > Furthermore, UFFDIO_MOVE enables moving swapped-out pages without > touching them within the same vma. Today, it can only be done by mremap, > however it forces splitting the vma. > > [1] https://lore.kernel.org/all/1425575884-2574-1-git-send-email-aarcange@redhat.com/ > [2] https://lore.kernel.org/linux-mm/CA+EESO4uO84SSnBhArH4HvLNhaUQ5nZKNKXqxRCyjniNVjp0Aw@mail.gmail.com/ > > Update for the ioctl_userfaultfd(2) manpage: > > UFFDIO_MOVE > (Since Linux xxx) Move a continuous memory chunk into the > userfault registered range and optionally wake up the blocked > thread. The source and destination addresses and the number of > bytes to move are specified by the src, dst, and len fields of > the uffdio_move structure pointed to by argp: > > struct uffdio_move { > __u64 dst; /* Destination of move */ > __u64 src; /* Source of move */ > __u64 len; /* Number of bytes to move */ > __u64 mode; /* Flags controlling behavior of move */ > __s64 move; /* Number of bytes moved, or negated error */ > }; > > The following value may be bitwise ORed in mode to change the > behavior of the UFFDIO_MOVE operation: > > UFFDIO_MOVE_MODE_DONTWAKE > Do not wake up the thread that waits for page-fault > resolution > > UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES > Allow holes in the source virtual range that is being moved. > When not specified, the holes will result in ENOENT error. > When specified, the holes will be accounted as successfully > moved memory. This is mostly useful to move hugepage aligned > virtual regions without knowing if there are transparent > hugepages in the regions or not, but preventing the risk of > having to split the hugepage during the operation. > > The move field is used by the kernel to return the number of > bytes that was actually moved, or an error (a negated errno- > style value). If the value returned in move doesn't match the > value that was specified in len, the operation fails with the > error EAGAIN. The move field is output-only; it is not read by > the UFFDIO_MOVE operation. > > The operation may fail for various reasons. Usually, remapping of > pages that are not exclusive to the given process fail; once KSM > might deduplicate pages or fork() COW-shares pages during fork() > with child processes, they are no longer exclusive. Further, the > kernel might only perform lightweight checks for detecting whether > the pages are exclusive, and return -EBUSY in case that check fails. > To make the operation more likely to succeed, KSM should be > disabled, fork() should be avoided or MADV_DONTFORK should be > configured for the source VMA before fork(). > > This ioctl(2) operation returns 0 on success. In this case, the > entire area was moved. On error, -1 is returned and errno is > set to indicate the error. Possible errors include: > > EAGAIN The number of bytes moved (i.e., the value returned in > the move field) does not equal the value that was > specified in the len field. > > EINVAL Either dst or len was not a multiple of the system page > size, or the range specified by src and len or dst and len > was invalid. > > EINVAL An invalid bit was specified in the mode field. > > ENOENT > The source virtual memory range has unmapped holes and > UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES is not set. > > EEXIST > The destination virtual memory range is fully or partially > mapped. > > EBUSY > The pages in the source virtual memory range are not > exclusive to the process. The kernel might only perform > lightweight checks for detecting whether the pages are > exclusive. To make the operation more likely to succeed, > KSM should be disabled, fork() should be avoided or > MADV_DONTFORK should be configured for the source virtual > memory area before fork(). > > ENOMEM Allocating memory needed for the operation failed. > > ESRCH > The faulting process has exited at the time of a > UFFDIO_MOVE operation. > A general comment simply because I realized that just now: does anything speak against limiting the operations now to a single MM? The use cases I heard so far don't need it. If ever required, we could consider extending it. Let's reduce complexity and KIS unless really required. Further: see "22) Do not crash the kernel" in coding-style.rst. All these BUG_ON need to go. Ideally, use WARN_ON_ONCE() or just VM_WARN_ON(). -- Cheers, David / dhildenb