On Thu, Apr 6, 2023 at 10:29 AM Peter Xu <peterx@xxxxxxxxxx> 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@xxxxxxxxxx/ > > 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@xxxxxxxxxxxx/ 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 >
