RE: [PATCH] mm/mremap: allow VMAs with VM_DONTEXPAND|VM_PFNMAP when creating new mapping

["Kasireddy, Vivek" <vivek.kasireddy@intel.com>]

> Subject: Re: [PATCH] mm/mremap: allow VMAs with
> VM_DONTEXPAND|VM_PFNMAP when creating new mapping
> 
> * David Hildenbrand (Red Hat) <david@kernel.org> [251121 03:13]:
> > >
> > > Another way round here might be to try to use userland to figure out
> the file
> > > that a mapping belongs to via e.g. /proc/$pid/map_files, determine the
> > > attributes from /proc/$pid/maps and then generate a new mmap() call
> with the
> > > same properties.
> >
> > Right, if we could find some way to just obtain the file/fd we could just
> > avoid mremap altogether.
> >
> > I recall that CRIU used something similar to obtain the fd of a
> > MAP_SHARED|MAP_ANON mapping (anon shmem) VMA, but my memory
> is a bit vague
> > on that one.
> 
> There is an ioctl query method to get information on VMAs since 6.11.
> 
> Check Documentation/filesystems/proc.rst for this:
> 
> Starting with 6.11 kernel, /proc/PID/maps provides an alternative
> ioctl()-based API that gives ability to flexibly and efficiently query and
> filter individual VMAs. This interface is binary and is meant for more
> efficient and easy programmatic use. `struct procmap_query`, defined in
> linux/fs.h UAPI header, serves as an input/output argument to the
> `PROCMAP_QUERY` ioctl() command. See comments in linus/fs.h UAPI
> header for
> details on query semantics, supported flags, data returned, and general API
> usage information.
> 
> 
> ... maybe that should be linux/fs.h and not linus/fs.h.  The actual file
> seems to be include/uapi/linux/fs.h (grepping for procmap_query).
> 
> Maybe you could make use of that ioctl to get what you want?
> 
> If not, maybe look at adding what you need there would be more of the
> change you want to make?
Thank you Liam, Lorenzo, David for your suggestions and comments.
We'll try other ways to address our use-case including exploring the
idea of obtaining the underlying fd via VFIO APIs or using
/proc/PID/maps.

Thanks,
Vivek

> 
> Thanks,
> Liam