Re: RFC for new feature to move pages from one vma to another without split

[Lokesh Gidra <lokeshgidra@google.com>]

On Thu, Apr 6, 2023 at 10:29 AM Peter Xu <peterx@redhat.com> wrote:
>
> Hi, Lokesh,
>
> Sorry for a late reply.  Copy Blake Caldwell and Mike too.

Thanks for the reply. It's extremely helpful.
>
> On Thu, Feb 16, 2023 at 02:27:11PM -0800, Lokesh Gidra wrote:
> > I) SUMMARY:
> > Requesting comments on a new feature which remaps pages from one
> > private anonymous mapping to another, without altering the vmas
> > involved. Two alternatives exist but both have drawbacks:
> > 1. userfaultfd ioctls allocate new pages, copy data and free the old
> > ones even when updates could be done in-place;
> > 2. mremap results in vma splitting in most of the cases due to 'pgoff' mismatch.
>
> Personally it was always a mistery to me on how vm_pgoff works with
> anonymous vmas and why it needs to be setup with vm_start >> PAGE_SHIFT.
>
> Just now I tried to apply below oneliner change:
>
> @@ -1369,7 +1369,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>                         /*
>                          * Set pgoff according to addr for anon_vma.
>                          */
> -                       pgoff = addr >> PAGE_SHIFT;
> +                       pgoff = 0;
>                         break;
>                 default:
>                         return -EINVAL;
>
> The kernel even boots without a major problem so far..
>
> I had a feeling that I miss something else here, it'll be great if anyone
> knows.
>
> Anyway, I agree mremap() is definitely not the best way to do page level
> operations like this, no matter whether vm_pgoff can match or not.
>
> >
> > Proposing a new mremap flag or userfaultfd ioctl which enables
> > remapping pages without these drawbacks. Such a feature, as described
> > below, would be very helpful in efficient implementation of concurrent
> > compaction algorithms.
>
> After I read the proposal, I had a feeling that you're not aware that we
> have similar proposals adding UFFDIO_REMAP.

Yes, I wasn't aware of this. Thanks a lot for sharing the details.
>
> I think it started with Andrea's initial proposal on the whole uffd:
>
> https://lore.kernel.org/linux-mm/1425575884-2574-1-git-send-email-aarcange@redhat.com/
>
> Then for some reason it's not merged in initial version, but at least it's
> been proposed again here (even though it seems the goal is slightly
> different; that may want to move page out instead of moving in):
>
> https://lore.kernel.org/linux-mm/cover.1547251023.git.blake.caldwell@colorado.edu/

Yeah, this seems to be the opposite of what I'm looking for. IIUC,
page out REMAP can't
satisfy any MISSING userfault. In fact, it enables MISSING faults in
future. Maybe a flag
can be added to uffdio_remap struct to accommodate this case, if it is
still being pursued.
>
> Also worth checking with the latest commit that Andrea maintains himself (I
> doubt whether there's major changes, but still just to make it complete):
>
> https://gitlab.com/aarcange/aa/-/commit/2aec7aea56b10438a3881a20a411aa4b1fc19e92
>
> So far I think that's what you're looking for. I'm not sure whether the
> limitations will be a problem, though, at least mentioned in the old
> proposals of UFFDIO_REMAP.  For example, it required not only anonymous but
> also mapcount==1 on all src pages.  But maybe that's not a problem here
> too.

Yes, this is exactly what I am looking for. The mapcount==1 is not a
problem either. Any idea why the patch isn't merged?

>
> >
> > II) MOTIVATION:
> > Garbage collectors (like the ones used in managed languages) perform
> > defragmentation of the managed heap by moving objects (of varying
> > sizes) within the heap. Usually these algorithms have to be concurrent
> > to avoid response time concerns. These are concurrent in the sense
> > that while the GC threads are compacting the heap, application threads
> > continue to make progress, which means enabling access to the heap
> > while objects are being simultaneously moved.
> >
> > Given the high overhead of heap compaction, such algorithms typically
> > segregate the heap into two types of regions (set of contiguous
> > pages): those that have enough fragmentation to compact, and those
> > that are densely populated. While only ‘fragmented’ regions are
> > compacted by sliding objects, both types of regions are traversed to
> > update references in them to the moved objects.
> >
> > A) PROT_NONE+SIGSEGV approach:
> > One of the widely used techniques to ensure data integrity during
> > concurrent compaction is to use page-level access interception.
> > Traditionally, this is implemented by mprotecting (PROT_NONE) the heap
> > before starting compaction and installing a SIGSEGV handler. When GC
> > threads are compacting the heap, if some application threads fault on
> > the heap, then they compact the faulted page in the SIGSEGV handler
> > and then enable access to it before returning. To do this atomically,
> > the heap must use shmem (MAP_SHARED) so that an alias mapping (with
> > read-write permission) can be used for moving objects into and
> > updating references.
> >
> > Limitation: due to different access rights, the heap can end up with
> > one vma per page in the worst case, hitting the ‘max_map_count’ limit.
> >
> > B) Userfaultfd approach:
> > Userfaultfd avoids the vma split issue by intercepting page-faults
> > when the page is missing and gives control to user-space to map the
> > desired content. It doesn’t affect the vma properties. The compaction
> > algorithm in this case works by first remapping the heap pages (using
> > mremap) to a secondary mapping and then registering the heap with
> > userfaultfd for MISSING faults. When an application thread accesses a
> > page that has not yet been mapped (by other GC/application threads), a
> > userfault occurs, and as a consequence the corresponding page is
> > generated and mapped using one of the following two ioctls.
> > 1) COPY ioctl: Typically the heap would be private anonymous in this
> > case. For every page on the heap, compact the objects into a
> > page-sized buffer, which COPY ioctl takes as input. The ioctl
> > allocates a new page, copies the input buffer to it, and then maps it.
> > This means that even for updating references in the densely populated
> > regions (where compaction is not done), in-place updation is
> > impossible. This results in unnecessary page-clear, memcpy and
> > freeing.
> > 2) CONTINUE ioctl: the two mappings (heap and secondary) are
> > MAP_SHARED to the same shmem file. Userfaults in the ‘fragmented’
> > regions are MISSING, in which case objects are compacted into the
> > corresponding secondary mapping page (which triggers a regular page
> > fault to get a page mapped) and then CONTINUE ioctl is invoked, which
> > maps the same page on the heap mapping. On the other hand, userfaults
> > in the ‘densely populated’ regions are MINOR (as the page already
> > exists in the secondary mapping), in which case we update the
> > references in the already existing page on the secondary mapping and
> > then invoke CONTINUE ioctl.
> >
> > Limitation: we observed in our implementation that
> > page-faults/page-allocation, memcpy, and madvise took (with either of
> > the two ioctls) ~50% of the time spent in compaction.
>
> I assume "page-faults" applies to CONTINUE, while "page-allocation" applies
> to COPY here.  UFFDIO_REMAP can definitely avoid memcpy, but I don't know
> how much it'll remove in total, e.g., I don't think page faults can be
> avoided anyway?  Also, madvise(), depending on what it is.  If it's only
> MADV_DONTNEED, maybe it'll be helpful too so the library can reuse wasted
> pages directly hence reducing DONTNEEDs.
>
That's right. page-faults -> CONTINUE and page-allocation -> COPY. The
GC algorithm
I'm describing here is mostly page-fault free as the heap pages are recycled.

Basically, the heap is mremapped to a secondary mapping so that we can
start receiving MISSING faults
on the heap after userfaultfd registration. Consequently, on every
MISSING userfault, the pages from the
secondary mapping are prepared in-place before acting as 'src' for
UFFDIO_REMAP ioctl call.

Also, as you said, MADV_DONTNEED will be mostly eliminated as most of
the pages are recycled in userspace.

There are other things too that UFFDIO_REMAP enables us to do. It
allows coarse-grained page-by-page compaction
of the heap without swapping-in the pages. This isn't possible today.

> > III) USE CASE (of the proposed feature):
> > The proposed feature of moving pages from one vma to another will
> > enable us to:
> > A) Recycle pages entirely in the userspace as they are freed (pages
> > whose objects are already consumed as part of the current compaction
> > cycle) in the ‘fragmented’ regions. This way we avoid page-clearing
> > (during page allocation) and memcpy (in the kernel). When the page is
> > handed over to the kernel for remapping, there is nothing else needed
> > to be done. Furthermore, since the page is being reused, it doesn’t
> > have to be freed either.
> > B) Implement a coarse-grained page-level compaction algorithm wherein
> > pages containing live objects are slid next to each other without
> > touching them, while reclaiming in-between pages which contain only
> > garbage. Such an algorithm is very useful for compacting objects which
> > are seldom accessed by application and hence are likely to be swapped
> > out. Without this feature, this would require copying the pages
> > containing live objects, for which the src pages have to be
> > swapped-in, only to be soon swapped-out afterwards.
> >
> > AFAIK, none of the above features can be implemented using mremap
> > (with current flags), irrespective of whether the heap is a shmem or
> > private anonymous mapping, because:
> > 1) When moving a page it’s likely that its index will need to change
> > and mremapping such a page would result in VMA splitting.
> > 2) Using mremap for moving pages would result in the heap’s range
> > being covered by several vmas. The mremap in the next compaction cycle
> > (required prior to starting compaction as described above), will fail
> > with EFAULT. This is because the src range in mremap is not allowed to
> > span multiple vmas. On the other hand, calling it for each src vma is
> > not feasible because:
> >   a) It’s not trivial to identify various vmas covering the heap range
> > in userspace, and
> >   b) This operation is supposed to happen with application threads
> > paused. Invoking numerous mremap syscalls in a pause risks causing
> > janks.
> > 3) Mremap has scalability concerns due to the need to acquire mmap_sem
> > exclusively for splitting/merging VMAs. This would impact parallelism
> > of application threads, particularly during the beginning of the
> > compaction process when they are expected to cause a spurt of
> > userfaults.
> >
> >
> > IV) PROPOSAL:
> > Initially, maybe the feature can be implemented only for private
> > anonymous mappings. There are two ways this can be implemented:
> > A) A new userfaultfd ioctl, ‘MOVE’, which takes the same inputs as the
> > ‘COPY’ ioctl. After sanity check, the ioctl would detach the pte
> > entries from the src vma, and move them to dst vma while updating
> > their ‘mapping’ and ‘index’ fields, if required.
> >
> > B) Add a new flag to mremap, ‘MREMAP_ONLYPAGES’, which works similar
> > to the MOVE ioctl above.
> >
> > Assuming (A) is implemented, here is broadly how the compaction would work:
> > * For a MISSING userfault in the ‘densely populated’ regions, update
> > pointers in-place in the secondary mapping page corresponding to the
> > fault address (on the heap) and then use the MOVE ioctl to map it on
> > the heap. In this case the ‘index’ field would remain the same.
> > * For a MISSING userfault in ‘fragmented’ regions, pick any freed page
> > in the secondary map, compact the objects corresponding to the fault
> > address in this page and then use MOVE ioctl to map it on the fault
> > address in the heap. This would require updating the ‘index’ field.
> > After compaction is completed, use madvise(MADV_DONTNEED) on the
> > secondary mapping to free any remaining pages.
> >
> >
> > Thanks,
> > Lokesh
> >
>
> --
> Peter Xu
>