Re: [PATCH v3 3/4] mm/page_owner: add NUMA node filter with nodelist support

[SeongJae Park <sj@kernel.org>]

On Thu, 30 Apr 2026 11:56:33 +0800 "zhen.ni" <zhen.ni@easystack.cn> wrote:

> 
> 
> 在 2026/4/29 22:56, SeongJae Park 写道:
> > On Wed, 29 Apr 2026 17:03:56 +0800 "zhen.ni" <zhen.ni@easystack.cn> wrote:
> > 
> >>
> >>
> >> 在 2026/4/29 09:28, SeongJae Park 写道:
> >>> On Tue, 28 Apr 2026 15:11:11 +0800 Zhen Ni <zhen.ni@easystack.cn> wrote:
> > [...]
> >>>> @@ -685,6 +685,7 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
> >>>>    	struct page_ext *page_ext;
> >>>>    	struct page_owner *page_owner;
> >>>>    	depot_stack_handle_t handle;
> >>>> +	nodemask_t mask;
> >>>>    
> >>>>    	if (!static_branch_unlikely(&page_owner_inited))
> >>>>    		return -EINVAL;
> >>>> @@ -698,6 +699,8 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
> >>>>    	while (!pfn_valid(pfn) && (pfn & (MAX_ORDER_NR_PAGES - 1)) != 0)
> >>>>    		pfn++;
> >>>>    
> >>>> +	mask = owner_filter.nid_mask;
> >>>> +
> >>>
> >>> READ_ONCE() was used for owner_filter.print_mode.  Should nid_mask also read
> >>> using READ_ONCE()?
> >>>
> >> The reason is that `owner_filter.nid_mask` is a nodemask_t, which is a
> >> 128-byte structure. READ_ONCE() only supports types up to 8 bytes and
> >> will trigger a compile-time assertion failure for larger structures.
> >>
> >> This was actually an issue in v2 - the AI review tool (sashiko.dev) and
> >> Andrew both caught the compilation error with READ_ONCE/WRITE_ONCE on
> >> nodemask_t, so v3 removed them.
> > 
> > Thank you for kindly sharing the context.  Now I understand why READ_ONCE()
> > cannot be used.  But, is plain load/store safe enough for nodemask_t?
> > Shouldn't it still be protected against races?
> > 
> Concurrency Safety:
> I considered spinlock and RCU, but decided against them:
> 
> - Spinlock: Adds overhead on every read, overkill for a debug facility
> - RCU: Requires dynamic allocation of 128-byte nodemask_t, too complex
> - READ_ONCE/WRITE_ONCE: Not possible, exceeds 8-byte limit
> 
> Plain load/store is safe here because:
> 1. page_owner is debug code with low-frequency filter changes
> 2. Worst case of torn read is temporary inconsistency in debug output
> 3. Similar debugfs interfaces use the same approach
> 
> The overhead of locking doesn't justify the benefit for this debug use case.
> 
> Do you think this is acceptable, or would you prefer I add locking?

Thank you for kindly explaining this.  Unless others have different opinions, I
think this is ok.  But, I think this would be good to be clarly documented, on
the code or the user documentation.

[...]
> >>>> +	/* Support: "-1" to clear, or nodelist format like "0", "0,2", "0-3" */
> >>>> +	if (kstrtoint(kbuf, 10, &val) == 0 && val == -1)
> >>>> +		nodes_clear(mask);
> >>>> +	else if (nodelist_parse(kbuf, mask)) {
> >>>> +		ret = -EINVAL;
> >>>> +		goto out_free;
> >>>> +	}
> >>>
> >>> Doesn't empty string input to nodelist_parse() clears the mask?  Can't it be
> >>> reused?
> >>>
> >> Yes, empty input (echo > nid) works because nodelist_parse() handles it
> >> correctly. However, nodelist_parse() - which is implemented via
> >> bitmap_parselist() - cannot handle "-1" as it's not a valid range format
> >> and would return an error. The explicit "-1" check is necessary to
> >> support `echo "-1" > nid` without returning an error.
> >>
> >> So the "-1" check handles a case that nodelist_parse() cannot handle.
> > 
> > Thank you for kindly explaining the reason.  But, do we really need to support
> > "-1" input?  Couldn't we just redefine the interface?
> > 
> I chose "-1" to clearly differentiate from valid NUMA node IDs (0, 1, 2, 
> 3...).Since node IDs are non-negative integers, "-1" naturally means 
> "invalid" or "no filter", which is an intuitive convention in Linux 
> (e.g., pid -1, signal -1).
> 
> Do you have a better suggestion for how to represent "clear filter"？

Seems my suggestion was too implicit.  I'm suggesting using empty string
instead of "-1".  I think it is also clarly differentiated from valid NUMA node
IDs?


Thanks,
SJ

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