Re: Re: Re: [PATCH v1 1/3] softmmu/physmem: fallback to opening guest RAM file as readonly in a MAP_PRIVATE mapping

[Peter Xu]

On Fri, Aug 11, 2023 at 01:49:52PM +0800, ThinerLogoer wrote:
> At 2023-08-11 05:24:43, "Peter Xu" <peterx@redhat.com> wrote:
> >On Fri, Aug 11, 2023 at 01:06:12AM +0800, ThinerLogoer wrote:
> >> >I think we have the following options (there might be more)
> >> >
> >> >1) This patch.
> >> >
> >> >2) New flag for memory-backend-file. We already have "readonly" and 
> >> >"share=". I'm having a hard time coming up with a good name that really 
> >> >describes the subtle difference.
> >> >
> >> >3) Glue behavior to the QEMU machine
> >> >
> >> 
> >> 4) '-deny-private-discard' argv, or environment variable, or both
> >
> >I'd personally vote for (2).  How about "fdperm"?  To describe when we want
> >to use different rw permissions on the file (besides the access permission
> >of the memory we already provided with "readonly"=XXX).  IIUC the only sane
> >value will be ro/rw/default, where "default" should just use the same rw
> >permission as the memory ("readonly"=XXX).
> >
> >Would that be relatively clean and also work in this use case?
> >
> >(the other thing I'd wish we don't have that fallback is, as long as we
> > have any of that "fallback" we'll need to be compatible with it since
> > then, and for ever...)
> 
> If it must be (2), I would vote (2) + (4), with (4) adjust the default 
> behavior of said `fdperm`.
> Mainly because (private+discard) is itself not a good practice and (4) serves
> as a good tool to help catch existing (private+discard) problems.
> 
> Actually (readonly+private) is more reasonable than (private+discard), so I
> want at least one room for a default (readonly+private) behavior.

Just for purely discussion purpose: I think maybe someday private+discard
could work.  IIUC what we're missing is an syscall interface to install a
zero page for a MAP_PRIVATE, atomically freeing what's underneath: it seems
either punching a hole or DONTNEED won't suffice here.  It'll just be
another problem when having zero page involved in file mappings at least.

> 
> Also in my case I kind of have to use "-mem-path" despite it being considered
> to be close to deprecated. Only with this I can avoid knowledge of memory
> backend before migration. Actually there seems to be no equivalent working 
> after-migration
> setup of "-object memory-backend-file,... -machine q35,mem=..." that can match
> before-migration setup of "-machine q35" (specifying nothing). Therefore
> I must make a plan and choose a migration method BEFORE I boot the
> machine and prepare to migrate, reducing the operation freedom.
> Considering that, I have to use "-mem-path" which keeps the freedom but
> has no configurable argument and I have to rely on default config.
> 
> Are there any "-object memory-backend-file..." setup equivalent to "-machine 
> q35"
> that can migrate from and to each other? If there is, I want to try it out.
> By the way "-object memory-backend-file,id=pc.ram" has just been killed by an 
> earlier
> commit.

I'm actually not familiar enough on the interfaces here, but I just checked
up the man page; would this work for you, together with option (2)?

        memory-backend='id'
                An alternative to legacy -mem-path and mem-prealloc options.  
Allows to use a memory backend as main RAM.

                For example:

                -object 
memory-backend-file,id=pc.ram,size=512M,mem-path=/hugetlbfs,prealloc=on,share=on
                -machine memory-backend=pc.ram
                -m 512M

-- 
Peter Xu