Re: [PATCH v7 09/15] util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERVE under Linux

[Daniel P . Berrangé]

On Wed, Apr 28, 2021 at 03:37:48PM +0200, David Hildenbrand wrote:
> Let's support RAM_NORESERVE via MAP_NORESERVE on Linux. The flag has no
> effect on most shared mappings - except for hugetlbfs and anonymous memory.
> 
> Linux man page:
>   "MAP_NORESERVE: Do not reserve swap space for this mapping. When swap
>   space is reserved, one has the guarantee that it is possible to modify
>   the mapping. When swap space is not reserved one might get SIGSEGV
>   upon a write if no physical memory is available. See also the discussion
>   of the file /proc/sys/vm/overcommit_memory in proc(5). In kernels before
>   2.6, this flag had effect only for private writable mappings."
> 
> Note that the "guarantee" part is wrong with memory overcommit in Linux.
> 
> Also, in Linux hugetlbfs is treated differently - we configure reservation
> of huge pages from the pool, not reservation of swap space (huge pages
> cannot be swapped).
> 
> The rough behavior is [1]:
> a) !Hugetlbfs:
> 
>   1) Without MAP_NORESERVE *or* with memory overcommit under Linux
>      disabled ("/proc/sys/vm/overcommit_memory == 2"), the following
>      accounting/reservation happens:
>       For a file backed map
>        SHARED or READ-only - 0 cost (the file is the map not swap)
>        PRIVATE WRITABLE - size of mapping per instance
> 
>       For an anonymous or /dev/zero map
>        SHARED   - size of mapping
>        PRIVATE READ-only - 0 cost (but of little use)
>        PRIVATE WRITABLE - size of mapping per instance
> 
>   2) With MAP_NORESERVE, no accounting/reservation happens.
> 
> b) Hugetlbfs:
> 
>   1) Without MAP_NORESERVE, huge pages are reserved.
> 
>   2) With MAP_NORESERVE, no huge pages are reserved.
> 
> Note: With "/proc/sys/vm/overcommit_memory == 0", we were already able
> to configure it for !hugetlbfs globally; this toggle now allows
> configuring it more fine-grained, not for the whole system.
> 
> The target use case is virtio-mem, which dynamically exposes memory
> inside a large, sparse memory area to the VM.

Can you explain this use case in more real world terms, as I'm not
understanding what a mgmt app would actually do with this in
practice ?




Regards,
Daniel
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