Messages in this thread |  | | From | David Hildenbrand <> | Subject | Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory | Date | Mon, 22 Feb 2021 16:30:47 +0100 |
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On 22.02.21 15:02, Michal Hocko wrote: > On Mon 22-02-21 14:22:37, David Hildenbrand wrote: >>>> Exactly. But for hugetlbfs/shmem ("!RAM-backed files") this is not what we >>>> want. >>> >>> OK, then I must have misread your requirements. Maybe I just got lost in >>> all the combinations you have listed. >> >> Another special case could be dax/pmem I think. You might want to fault it >> in readable/writable but not perform an actual read/write unless really >> required. >> >> QEMU phrases this as "don't cause wear on the storage backing". > > Sorry for being dense here but I still do not follow. If you do not want > to read then what do you want to populate from? Only map if it is in the
In the context of VMs it's usually rather a mean to preallocate backend storage - which would also happen on read access. See below on case 4).
> page cache?
Let's try to untangle my thoughts regarding VMs. We could have as backend storage for our VM:
1) Anonymous memory 2) hugetlbfs (private/shared) 3) tmpfs/shmem (private/shared) 4) Ordinary files (shared) 5) DAX/PMEM (shared)
Excluding special cases (hypervisor upgrades with 2) and 3) ), we expect to have pre-existing content in files only in 4) and 5). 4) and 5) might be used as NVDIMM backend for a guest, or as DIMM backend.
The first access of our VM to memory could be a) Write: the usual case when exposed as RAM/DIMM to out guest. b) Read: possible case when exposed as an NVDIMM to our guest (we don't know). But eventually, we might write to (parts of) NVDIMMs later.
We "preallocate"/"populate" memory of our VM so that - We know we have sufficient backend storage (esp. hugetlbfs, shmem, files) - so we don't randomly crash the VM. My most important use case. - We avoid page faults (including page zeroing!) at runtime. Especially relevant for RT workloads.
With 1), 2), and 3) we want to have pages faulted in writable - we expect that our guest will write to that memory. MADV_POPULATE would do that only for 1), and MAP_PRIVATE of 2). For the shared parts, we would want MADV_POPULATE_WRITE semantics.
With 5), we already had complaints that preallcoation in QEMU takes a long time - because we end up actually reading/writing slow PMEM (libvirt now disables preallcoation for that reason, which makes sense). However, MADV_POPULATE_WRITE would help to prefault without actually reading/writing pmem - if we want to avoid any minor faults.
With 4), I think we primarily prealloc/prefault to make sure we have sufficient backend storage. fallocate() might do a better job just for the allocation. But if there is sufficient RAM it might make sense to prefault all guest RAM at least readable - then we only have a minor fault when the VM writes to it and might avoid having to go to disk. Prefaulting everything writable means that we *have to* write back all guest RAM even if the guest never accessed it. So I think there are cases where MADV_POPULATE_READ (current MADV_POPULATE) semantics could make sense.
-- Thanks,
David / dhildenb
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