Re: [PATCH 1/3] mm: introduce snapshot_page()

[Luiz Capitulino <luizcap@redhat.com>]

On 2025-06-26 17:39, Andrew Morton wrote:
> On Thu, 26 Jun 2025 14:16:51 -0400 Luiz Capitulino <luizcap@redhat.com> wrote:
> 
>> This commit refactors __dump_page() into snapshot_page().
>>
>> snapshot_page() tries to take a faithful snapshot of a page and its
>> folio representation. The snapshot is returned in the struct
>> page_snapshot parameter along with additional flags that are best
>> retrieved at snapshot creation time to reduce race windows.
>>
>> This function is intended to be used by callers that need a stable
>> representation of a struct page and struct folio so that pointers
>> or page information doesn't change while working on a page.
>>
>> The idea and original implemenetation of snapshot_page() comes from
> 
> tpyo!
> 
>> Matthew Wilcox with suggestions for improvements from David Hildenbrand.
>> All bugs and misconceptions are mine.
>>
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -4184,4 +4184,24 @@ static inline bool page_pool_page_is_pp(struct page *page)
>>   }
>>   #endif
>>   
>> +#define PAGE_SNAPSHOT_FAITHFUL     (1 << 0)
>> +#define PAGE_SNAPSHOT_PG_HUGE_ZERO (1 << 1)
>> +#define PAGE_SNAPSHOT_PG_FREE      (1 << 2)
>> +#define PAGE_SNAPSHOT_PG_IDLE      (1 << 3)
>> +
>> +struct page_snapshot {
>> +	struct folio folio_snapshot;
>> +	struct page page_snapshot;
>> +	unsigned long pfn;
>> +	unsigned long idx;
>> +	unsigned long flags;
>> +};
>> +
>> +static inline bool snapshot_page_is_faithful(const struct page_snapshot *ps)
>> +{
>> +	return ps->flags & 0x1;'
> 
> 	& PAGE_SNAPSHOT_FAITHFUL?
> 
>> +}
>> +
> 
> All looks sane to me.  Small-system people (are there any left?) might
> point out that all the new code could be under ifdef CONFIG_PROCFS?
> 
> I'll skip v1, see what reviewers have to say, thanks.

Yes, no rush. And I'll fix the things you pointed out for v2 (including
ifdef CONFIG_PROCFS). Thanks for the super quick feedback.