Re: [PATCH V3 01/16] machine: anon-alloc option

[Peter Xu <peterx@redhat.com>]

On Wed, Nov 06, 2024 at 03:12:20PM -0500, Steven Sistare wrote:
> 
> 
> On 11/4/2024 4:36 PM, David Hildenbrand wrote:
> > On 04.11.24 21:56, Steven Sistare wrote:
> > > On 11/4/2024 3:15 PM, David Hildenbrand wrote:
> > > > On 04.11.24 20:51, David Hildenbrand wrote:
> > > > > On 04.11.24 18:38, Steven Sistare wrote:
> > > > > > On 11/4/2024 5:39 AM, David Hildenbrand wrote:
> > > > > > > On 01.11.24 14:47, Steve Sistare wrote:
> > > > > > > > Allocate anonymous memory using mmap MAP_ANON or memfd_create depending
> > > > > > > > on the value of the anon-alloc machine property.  This option applies to
> > > > > > > > memory allocated as a side effect of creating various devices. It does
> > > > > > > > not apply to memory-backend-objects, whether explicitly specified on
> > > > > > > > the command line, or implicitly created by the -m command line option.
> > > > > > > > 
> > > > > > > > The memfd option is intended to support new migration modes, in which the
> > > > > > > > memory region can be transferred in place to a new QEMU process, by sending
> > > > > > > > the memfd file descriptor to the process.  Memory contents are preserved,
> > > > > > > > and if the mode also transfers device descriptors, then pages that are
> > > > > > > > locked in memory for DMA remain locked.  This behavior is a pre-requisite
> > > > > > > > for supporting vfio, vdpa, and iommufd devices with the new modes.
> > > > > > > 
> > > > > > > A more portable, non-Linux specific variant of this will be using shm,
> > > > > > > similar to backends/hostmem-shm.c.
> > > > > > > 
> > > > > > > Likely we should be using that instead of memfd, or try hiding the
> > > > > > > details. See below.
> > > > > > 
> > > > > > For this series I would prefer to use memfd and hide the details.  It's a
> > > > > > concise (and well tested) solution albeit linux only.  The code you supply
> > > > > > for posix shm would be a good follow on patch to support other unices.
> > > > > 
> > > > > Unless there is reason to use memfd we should start with the more
> > > > > generic POSIX variant that is available even on systems without memfd.
> > > > > Factoring stuff out as I drafted does look quite compelling.
> > > > > 
> > > > > I can help with the rework, and send it out separately, so you can focus
> > > > > on the "machine toggle" as part of this series.
> > > > > 
> > > > > Of course, if we find out we need the memfd internally instead under
> > > > > Linux for whatever reason later, we can use that instead.
> > > > > 
> > > > > But IIUC, the main selling point for memfd are additional features
> > > > > (hugetlb, memory sealing) that you aren't even using.
> > > > 
> > > > FWIW, I'm looking into some details, and one difference is that shmem_open() under Linux (glibc) seems to go to /dev/shmem and memfd/SYSV go to the internal tmpfs mount. There is not a big difference, but there can be some difference (e.g., sizing of the /dev/shm mount).
> > > 
> > > Sizing is a non-trivial difference.  One can by default allocate all memory using memfd_create.
> > > To do so using shm_open requires configuration on the mount.  One step harder to use.
> > 
> > Yes.
> > 
> > > 
> > > This is a real issue for memory-backend-ram, and becomes an issue for the internal RAM
> > > if memory-backend-ram has hogged all the memory.
> > > 
> > > > Regarding memory-backend-ram,share=on, I assume we can use memfd if available, but then fallback to shm_open().
> > > 
> > > Yes, and if that is a good idea, then the same should be done for internal RAM
> > > -- memfd if available and fallback to shm_open.
> > 
> > Yes.
> > 
> > > 
> > > > I'm hoping we can find a way where it just all is rather intuitive, like
> > > > 
> > > > "default-ram-share=on": behave for internal RAM just like "memory-backend-ram,share=on"
> > > > 
> > > > "memory-backend-ram,share=on": use whatever mechanism we have to give us "anonymous" memory that can be shared using an fd with another process.
> > > > 
> > > > Thoughts?
> > > 
> > > Agreed, though I thought I had already landed at the intuitive specification in my patch.
> > > The user must explicitly configure memory-backend-* to be usable with CPR, and anon-alloc
> > > controls everything else.  Now we're just riffing on the details: memfd vs shm_open, spelling
> > > of options and words to describe them.
> > 
> > Well, yes, and making it all a bit more consistent and the "machine option" behave just like "memory-backend-ram,share=on".
> 
> Hi David and Peter,
> 
> I have implemented and tested the following, for both qemu_memfd_create
> and qemu_shm_alloc.  This is pseudo-code, with error conditions omitted
> for simplicity.

I'm ok with either shm or memfd, as this feature only applies to Linux
anyway.  I'll leave that part to you and David to decide.

> 
> Any comments before I submit a complete patch?
> 
> ----
> qemu-options.hx:
>     ``aux-ram-share=on|off``
>         Allocate auxiliary guest RAM as an anonymous file that is
>         shareable with an external process.  This option applies to
>         memory allocated as a side effect of creating various devices.
>         It does not apply to memory-backend-objects, whether explicitly
>         specified on the command line, or implicitly created by the -m
>         command line option.
> 
>         Some migration modes require aux-ram-share=on.
> 
> qapi/migration.json:
>     @cpr-transfer:
>          ...
>          Memory-backend objects must have the share=on attribute, but
>          memory-backend-epc is not supported.  The VM must be started
>          with the '-machine aux-ram-share=on' option.
> 
> Define RAM_PRIVATE
> 
> Define qemu_shm_alloc(), from David's tmp patch
> 
> ram_backend_memory_alloc()
>     ram_flags = backend->share ? RAM_SHARED : RAM_PRIVATE;
>     memory_region_init_ram_flags_nomigrate(ram_flags)

Looks all good until here.

> 
> qemu_ram_alloc_internal()
>     ...
>     if (!host && !(ram_flags & RAM_PRIVATE) && current_machine->aux_ram_share)

Nitpick: could rely on flags-only, rather than testing "!host", AFAICT
that's equal to RAM_PREALLOC.  Meanwhile I slightly prefer we don't touch
anything if SHARED|PRIVATE is set.  All combined, it could be:

    if (!(ram_flags & (RAM_PREALLOC | RAM_PRIVATE | RAM_SHARED))) {
        // ramblock to be allocated, with no share/private request, aka,
        // aux memory chunk...
    }

>         new_block->flags |= RAM_SHARED;
> 
>     if (!host && (new_block->flags & RAM_SHARED)) {
>         qemu_ram_alloc_shared(new_block);

I'm not sure whether this needs its own helper.  Should we fallback to
ram_block_add() below, just like a RAM_SHARED?

IIUC, we could start to create RAM_SHARED in qemu_anon_ram_alloc() and
always cache the fd (even if we don't do that before)?

>     } else
>         new_block->fd = -1;
>         new_block->host = host;
>     }
>     ram_block_add(new_block);
> 
> qemu_ram_alloc_shared()
>     if qemu_memfd_check()
>         new_block->fd = qemu_memfd_create()
>     else
>         new_block->fd = qemu_shm_alloc()
>     new_block->host = file_ram_alloc(new_block->fd)
> ----
> 
> - Steve
> 

-- 
Peter Xu