linux-kernel - Re: [RFC PATCH 3/3] mm/migrate: Create move_phys

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Message-ID: <ZP2tYY00/q9ElFQn@memverge.com>
Date:   Sun, 10 Sep 2023 07:49:53 -0400
From:   Gregory Price <gregory.price@...verge.com>
To:     Jonathan Corbet <corbet@....net>
Cc:     Gregory Price <gourry.memverge@...il.com>,
        linux-mm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        linux-cxl@...r.kernel.org, luto@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, arnd@...db.de, akpm@...ux-foundation.org,
        x86@...nel.org
Subject: Re: [RFC PATCH 3/3] mm/migrate: Create move_phys_pages syscall

On Sun, Sep 10, 2023 at 02:36:40PM -0600, Jonathan Corbet wrote:
> Gregory Price <gourry.memverge@...il.com> writes:
> 
> > Similar to the move_pages system call, instead of taking a pid and
> > list of virtual addresses, this system call takes a list of physical
> > addresses.
> >
> > Because there is no task to validate the memory policy against, each
> > page needs to be interrogated to determine whether the migration is
> > valid, and all tasks that map it need to be interrogated.
> >
> > This is accomplished via an rmap_walk on the folio containing
> > the page, and interrogating all tasks that map the page.
> >
> > Each page must be interrogated individually, which should be
> > considered when using this to migrate shared regions.
> >
> > The remaining logic is the same as the move_pages syscall. One
> > change to do_pages_move is made (to check whether an mm_struct is
> > passed) in order to re-use the existing migration code.
> >
> > Signed-off-by: Gregory Price <gregory.price@...verge.com>
> > ---
> >  arch/x86/entry/syscalls/syscall_32.tbl  |   1 +
> >  arch/x86/entry/syscalls/syscall_64.tbl  |   1 +
> >  include/linux/syscalls.h                |   5 +
> >  include/uapi/asm-generic/unistd.h       |   8 +-
> >  kernel/sys_ni.c                         |   1 +
> >  mm/migrate.c                            | 178 +++++++++++++++++++++++-
> >  tools/include/uapi/asm-generic/unistd.h |   8 +-
> >  7 files changed, 197 insertions(+), 5 deletions(-)
> 
> So this is probably a silly question, but just to be sure ... what is
> the permission model for this system call?  As far as I can tell, the
> ability to move pages is entirely unrestricted, with the exception of
> pages that would need MPOL_MF_MOVE_ALL.  If so, that seems undesirable,
> but probably I'm just missing something ... ?
> 
> Thanks,
> 
> jon

Not silly, looks like when U dropped the CAP_SYS_NICE check (no task to
check against), check i neglected to add a CAP_SYS_ADMIN check.

Oversight on my part, I'll work it in with other feedback.

Thanks!
~Gregory