linux-kernel - Re: [RFC PATCH] mm/mbind: Introduce process_mbind() syscall for external memory binding

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Message-ID: <ZV/HSFMmv3xwkNPL@memverge.com>
Date:   Thu, 23 Nov 2023 16:42:32 -0500
From:   Gregory Price <gregory.price@...verge.com>
To:     Zhongkun He <hezhongkun.hzk@...edance.com>
Cc:     Vinicius Petrucci <vpetrucci@...il.com>, akpm@...ux-foundation.org,
        linux-mm@...r.kernel.org, linux-cxl@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-api@...r.kernel.org, minchan@...nel.org,
        dave.hansen@...ux.intel.com, x86@...nel.org,
        Jonathan.Cameron@...wei.com, aneesh.kumar@...ux.ibm.com,
        ying.huang@...el.com, dan.j.williams@...el.com, fvdl@...gle.com,
        surenb@...gle.com, rientjes@...gle.com, hannes@...xchg.org,
        mhocko@...e.com, Hasan.Maruf@....com, jgroves@...ron.com,
        ravis.opensrc@...ron.com, sthanneeru@...ron.com,
        emirakhur@...ron.com, vtavarespetr@...ron.com
Subject: Re: [RFC PATCH] mm/mbind: Introduce process_mbind() syscall for
 external memory binding

On Fri, Nov 24, 2023 at 04:13:41PM +0800, Zhongkun He wrote:
> 
> Per my understanding,  the process_mbind() is implementable without
> many difficult challenges，
> since it is always protected by mm->mmap_lock. But task mempolicy does
> not acquire any lock
> in alloc_pages().

per-vma policies are protected by the mmap lock, while the task
mempolicy is protected by the task lock on replacement, and
task->mems_allowed (protected by task_lock).

There is an update in my refactor tickets that enforces the acquisition
of task_lock on mpol_set_nodemask, which prevents the need for
alloc_pages to do anything else.  That's not present in this patch.

Basically mems_allowed deals with the majority of situations, and
mmap_lock deals with per-vma mempolicy changes and migrations.

~Gregory