Messages in this thread |  | | From | David Hildenbrand <> | Subject | Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory | Date | Sat, 20 Feb 2021 10:12:26 +0100 |
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On 17.02.21 16:48, David Hildenbrand wrote: > When we manage sparse memory mappings dynamically in user space - also > sometimes involving MADV_NORESERVE - we want to dynamically populate/ > discard memory inside such a sparse memory region. Example users are > hypervisors (especially implementing memory ballooning or similar > technologies like virtio-mem) and memory allocators. In addition, we want > to fail in a nice way if populating does not succeed because we are out of > backend memory (which can happen easily with file-based mappings, > especially tmpfs and hugetlbfs). > > While MADV_DONTNEED and FALLOC_FL_PUNCH_HOLE provide us ways to reliably > discard memory, there is no generic approach to populate ("preallocate") > memory. > > Although mmap() supports MAP_POPULATE, it is not applicable to the concept > of sparse memory mappings, where we want to do populate/discard > dynamically and avoid expensive/problematic remappings. In addition, > we never actually report error during the final populate phase - it is > best-effort only. > > fallocate() can be used to preallocate file-based memory and fail in a safe > way. However, it is less useful for private mappings on anonymous files > due to COW semantics. For example, using fallocate() to preallocate memory > on an anonymous memfd files that are mapped MAP_PRIVATE results in a double > memory consumption when actually writing via the mapping. In addition, > fallocate() does not actually populate page tables, so we still always > have to resolve minor faults on first access. > > Because we don't have a proper interface, what applications > (like QEMU and databases) end up doing is touching (i.e., writing) all > individual pages. However, it requires expensive signal handling (SIGBUS); > for example, this is problematic in hypervisors like QEMU where SIGBUS > handlers might already be used by other subsystems concurrently to e.g, > handle hardware errors. "Simply" doing preallocation from another thread > is not that easy. > > Let's introduce MADV_POPULATE with the following semantics > 1. MADV_POPULATED does not work on PROT_NONE and special VMAs. It works > on everything else. > 2. Errors during MADV_POPULATED (especially OOM) are reported. If we hit > hardware errors on pages, ignore them - nothing we really can or > should do. > 3. On errors during MADV_POPULATED, some memory might have been > populated. Callers have to clean up if they care. > 4. Concurrent changes to the virtual memory layour are tolerated - we > process each and every PFN only once, though. > 5. If MADV_POPULATE succeeds, all memory in the range can be accessed > without SIGBUS. (of course, not if user space changed mappings in the > meantime or KSM kicked in on anonymous memory). > > Although sparse memory mappings are the primary use case, this will > also be useful for ordinary preallocations where MAP_POPULATE is not > desired (e.g., in QEMU, where users can trigger preallocation of > guest RAM after the mapping was created). > > Looking at the history, MADV_POPULATE was already proposed in 2013 [1], > however, the main motivation back than was performance improvements > (which should also still be the case, but it's a seconary concern). > > Basic functionality was tested with: > - anonymous memory > - MAP_PRIVATE on anonymous file via memfd > - MAP_SHARED on anonymous file via memf > - MAP_PRIVATE on anonymous hugetlbfs file via memfd > - MAP_SHARED on anonymous hugetlbfs file via memfd > - MAP_PRIVATE on tmpfs/shmem file (we end up with double memory consumption > though, as the actual file gets populated with zeroes) > - MAP_SHARED on tmpfs/shmem file > > Note: For populating/preallocating zeroed-out memory while userfaultfd is > active, it's even faster to use first fallocate() or placing zeroed pages > via userfaultfd APIs. Otherwise, we'll have to route every fault while > populating via the userfaultfd handler. > > [1] https://lkml.org/lkml/2013/6/27/698 > > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Arnd Bergmann <arnd@arndb.de> > Cc: Michal Hocko <mhocko@suse.com> > Cc: Oscar Salvador <osalvador@suse.de> > Cc: Matthew Wilcox (Oracle) <willy@infradead.org> > Cc: Andrea Arcangeli <aarcange@redhat.com> > Cc: Minchan Kim <minchan@kernel.org> > Cc: Jann Horn <jannh@google.com> > Cc: Jason Gunthorpe <jgg@ziepe.ca> > Cc: Dave Hansen <dave.hansen@intel.com> > Cc: Hugh Dickins <hughd@google.com> > Cc: Rik van Riel <riel@surriel.com> > Cc: Michael S. Tsirkin <mst@redhat.com> > Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> > Cc: Vlastimil Babka <vbabka@suse.cz> > Cc: Richard Henderson <rth@twiddle.net> > Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru> > Cc: Matt Turner <mattst88@gmail.com> > Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de> > Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com> > Cc: Helge Deller <deller@gmx.de> > Cc: Chris Zankel <chris@zankel.net> > Cc: Max Filippov <jcmvbkbc@gmail.com> > Cc: linux-alpha@vger.kernel.org > Cc: linux-mips@vger.kernel.org > Cc: linux-parisc@vger.kernel.org > Cc: linux-xtensa@linux-xtensa.org > Cc: linux-arch@vger.kernel.org > Signed-off-by: David Hildenbrand <david@redhat.com> > --- > > If we agree that this makes sense I'll do more testing to see if we > are missing any return value handling and prepare a man page update to > document the semantics. > > Thoughts?
Thinking about MADV_POPULATE vs. MADV_POPULATE_WRITE I wonder if it would be more versatile to break with existing MAP_POPULATE semantics and directly go with
MADV_POPULATE_READ: simulate user space read access without actually reading. Trigger a read fault if required.
MADV_POPULATE_WRITE: simulate user space write access without actually writing. Trigger a write fault if required.
For my use case, I could use MADV_POPULATE_WRITE on anonymous memory and RAM-backed files (shmem/hugetlb) - I would not have a minor fault when the guest inside the VM first initializes memory. This mimics how QEMU currently preallocates memory.
However, I would use MADV_POPULATE_READ on any !RAM-backed files where we actually have to write-back to a (slow?) device. Dirtying everything although the guest might not actually consume it in the near future might be undesired.
MADV_POPULATE_READ could also come in handy in combination with userfaulfd-wp() [1], when handling unpopulated memory via ordinary userfaultfd MISSING events in undesired. I could imagine it can speed up live migration of VMs in general, where we might end up reading a lot of unpopulated memory to figure out it's all zeroes after faulting in the shared zeropage. Especially relevant with a shared zeropage.
Thoughts?
[1] https://lkml.kernel.org/r/20210219211054.GL6669@xz-x1
-- Thanks,
David / dhildenb
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