Re: [RFC PATCH 00/18] use struct ptdesc to replace pgtable_t

Alex Shi
2024-08-15T06:17:59Z

On 8/15/24 1:13 AM, Matthew Wilcox wrote:
> On Tue, Jul 30, 2024 at 02:46:54PM +0800, alexs@kernel.org wrote:
>> We have struct ptdesc for page table descriptor a year ago, but it
>> has no much usages in kernel, while pgtable_t is used widely.
>
> Hum, I thought I responded to this to point out the problem, but
> I don't see the response anywhere, so I'll try again.
>
>> The pgtable_t is typedefed as 'pte_t *' in sparc, s390, powerpc and m68k
>> except SUN3, others archs are all same as 'struct page *'.
>
> And there's a very good reason for that. On s390 and powerpc (I cannot
> speak to the sparc/m68k), each page table is (potentially) smaller
> than PAGE_SIZE. So we cannot do what your patch purports to do, as
> we would not know whether we're referring to the first or subsequent
> page tables contained within a page.
>> Maybe at some point in the distant future we'll be able to allocate
> a ptdesc per page table instead of per page allocated for use by page
> tables. But we cannot do that yet.

Got it. Thanks a lot for the info!