linux-kernel - Re: [PATCH v5] mm/ksm: introduce ksm

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Date:   Sun, 8 May 2022 19:03:36 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     cgel.zte@...il.com
Cc:     akpm@...ux-foundation.org, keescook@...omium.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, ran.xiaokai@....com.cn, wang.yong12@....com.cn,
        xu.xin16@....com.cn, yang.yang29@....com.cn,
        zhang.yunkai@....com.cn
Subject: Re: [PATCH v5] mm/ksm: introduce ksm_force for each process

On Sun, May 08, 2022 at 09:27:10AM +0000, cgel.zte@...il.com wrote:
> If ksm_force is set to 0, cancel the feature of ksm_force of this
> process and unmerge those merged pages belonging to VMAs which is not
> madvised as MADV_MERGEABLE of this process, but leave MADV_MERGEABLE
> areas merged.

Is that actually a useful feature?  Otherwise, we could simply turn
on/off the existing MMF_VM_MERGEABLE flag instead of introducing this
new bool.

> +Controlling KSM with procfs
> +===========================
> +
> +KSM can also operate on anonymous areas of address space of those processes's
> +knob ``/proc/<pid>/ksm_force`` is on, even if app codes doesn't call madvise()
> +explicitly to advise specific areas as MADV_MERGEABLE.
> +
> +You can set ksm_force to 1 to force all anonymous and qualified VMAs of
> +this process to be involved in KSM scanning. But It is effective only when the
> +klob of ``/sys/kernel/mm/ksm/run`` is set as 1.

I think that last sentence doesn't really add any value.

> +	memset(buffer, 0, sizeof(buffer));
> +	if (count > sizeof(buffer) - 1)
> +		count = sizeof(buffer) - 1;
> +	if (copy_from_user(buffer, buf, count)) {
> +		err = -EFAULT;
> +		goto out_return;

This feels a bit unnecessary.  Just 'return -EFAULT' here.

> +	}
> +
> +	err = kstrtoint(strstrip(buffer), 0, &force);
> +
> +	if (err)
> +		goto out_return;

'return err'

> +	if (force != 0 && force != 1) {
> +		err = -EINVAL;
> +		goto out_return;

'return -EINVAL'

> +	}
> +
> +	task = get_proc_task(file_inode(file));
> +	if (!task) {
> +		err = -ESRCH;
> +		goto out_return;

'return -ESRCH'