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Message-ID: <CA+EESO47LqwMwGgkHQdx1cBdcn_+FWqda8OPcBU-skk9yML_qA@mail.gmail.com>
Date: Mon, 9 Oct 2023 10:56:50 -0700
From: Lokesh Gidra <lokeshgidra@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
viro@...iv.linux.org.uk, brauner@...nel.org, shuah@...nel.org,
aarcange@...hat.com, peterx@...hat.com, hughd@...gle.com,
mhocko@...e.com, axelrasmussen@...gle.com, rppt@...nel.org,
willy@...radead.org, Liam.Howlett@...cle.com, jannh@...gle.com,
zhangpeng362@...wei.com, bgeffon@...gle.com,
kaleshsingh@...gle.com, ngeoffray@...gle.com, jdduke@...gle.com,
linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
kernel-team@...roid.com
Subject: Re: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI
On Mon, Oct 9, 2023 at 9:29 AM Lokesh Gidra <lokeshgidra@...gle.com> wrote:
>
> On Mon, Oct 9, 2023 at 5:24 PM David Hildenbrand <david@...hat.com> wrote:
> >
> > On 09.10.23 18:21, Suren Baghdasaryan wrote:
> > > On Mon, Oct 9, 2023 at 7:38 AM David Hildenbrand <david@...hat.com> wrote:
> > >>
> > >> On 09.10.23 08:42, Suren Baghdasaryan wrote:
> > >>> From: Andrea Arcangeli <aarcange@...hat.com>
> > >>>
> > >>> 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.
> > >
> > > Let me check if there are use cases that require moves between MMs.
> > > Andrea seems to have put considerable effort to make it work between
> > > MMs and it would be a pity to lose that. I can send a follow-up patch
> > > to recover that functionality and even if it does not get merged, it
> > > can be used in the future as a reference. But first let me check if we
> > > can drop it.
>
> For the compaction use case that we have it's fine to limit it to
> single MM. However, for general use I think Peter will have a better
> idea.
> >
> > Yes, that sounds reasonable. Unless the big important use cases requires
> > moving pages between processes, let's leave that as future work for now.
> >
> > --
> > Cheers,
> >
> > David / dhildenb
> >
While going through mremap's move_page_tables code, which is pretty
similar to what we do here, I noticed that cache is flushed as well,
whereas we are not doing that here. Is that OK? I'm not a MM expert by
any means, so it's a question rather than a comment :)
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