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

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Message-ID: <878r9dzrxj.fsf@meer.lwn.net>
Date:   Sun, 10 Sep 2023 14:36:40 -0600
From:   Jonathan Corbet <corbet@....net>
To:     Gregory Price <gourry.memverge@...il.com>, linux-mm@...r.kernel.org
Cc:     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,
        Gregory Price <gregory.price@...verge.com>
Subject: Re: [RFC PATCH 3/3] mm/migrate: Create move_phys_pages syscall

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