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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to
 prefault/prealloc memory
Message-ID: <7d7d2213-92a4-0419-20ad-bba7071a279c@redhat.com>
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