linux-kernel - Re: [PATCH v6] mm/ksm: introduce ksm

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Date:   Thu, 12 May 2022 22:07:09 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     cgel.zte@...il.com
Cc:     akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, corbet@....net,
        xu xin <xu.xin16@....com.cn>,
        Yang Yang <yang.yang29@....com.cn>,
        Ran Xiaokai <ran.xiaokai@....com.cn>,
        wangyong <wang.yong12@....com.cn>,
        Yunkai Zhang <zhang.yunkai@....com.cn>
Subject: Re: [PATCH v6] mm/ksm: introduce ksm_force for each process

On Tue, May 10, 2022 at 12:22:42PM +0000, cgel.zte@...il.com wrote:
> +++ b/Documentation/admin-guide/mm/ksm.rst
> @@ -32,7 +32,7 @@ are swapped back in: ksmd must rediscover their identity and merge again).
>  Controlling KSM with madvise
>  ============================
>  
> -KSM only operates on those areas of address space which an application
> +KSM can operates on those areas of address space which an application

"can operate on"

> +static ssize_t ksm_force_write(struct file *file, const char __user *buf,
> +				size_t count, loff_t *ppos)
> +{
> +	struct task_struct *task;
> +	struct mm_struct *mm;
> +	char buffer[PROC_NUMBUF];
> +	int force;
> +	int err = 0;
> +
> +	memset(buffer, 0, sizeof(buffer));
> +	if (count > sizeof(buffer) - 1)
> +		count = sizeof(buffer) - 1;
> +	if (copy_from_user(buffer, buf, count))
> +		return -EFAULT;
> +
> +	err = kstrtoint(strstrip(buffer), 0, &force);
> +	if (err)
> +		return err;
> +
> +	if (force != 0 && force != 1)
> +		return -EINVAL;
> +
> +	task = get_proc_task(file_inode(file));
> +	if (!task)
> +		return -ESRCH;
> +
> +	mm = get_task_mm(task);
> +	if (!mm)
> +		goto out_put_task;
> +
> +	if (mm->ksm_force != force) {
> +		if (mmap_write_lock_killable(mm)) {
> +			err = -EINTR;
> +			goto out_mmput;
> +		}
> +
> +		if (force == 0)
> +			mm->ksm_force = force;
> +		else {
> +			/*
> +			 * Force anonymous pages of this mm to be involved in KSM merging
> +			 * without explicitly calling madvise.
> +			 */
> +			if (!test_bit(MMF_VM_MERGEABLE, &mm->flags))
> +				err = __ksm_enter(mm);
> +			if (!err)
> +				mm->ksm_force = force;
> +		}
> +
> +		mmap_write_unlock(mm);
> +	}

There's a much simpler patch hiding inside this complicated one.

	if (force) {
		set_bit(MMF_VM_MERGEABLE, &mm->flags));
		for each VMA
			set VM_MERGEABLE;
		err = __ksm_enter(mm);
	} else {
		clear_bit(MMF_VM_MERGEABLE, &mm->flags));
		for each VMA
			clear VM_MERGEABLE;
	}

... and all the extra complications you added go away.