Re: [PATCH RFC 06/12] mm/gup: Drop folio_fast_pin_allowed() in hugepd processing

[Ryan Roberts <ryan.roberts@arm.com>]

On 04/12/2023 11:25, Christophe Leroy wrote:
> 
> 
> Le 04/12/2023 à 12:11, Ryan Roberts a écrit :
>> On 03/12/2023 13:33, Christophe Leroy wrote:
>>>
>>>
>>> Le 30/11/2023 à 22:30, Peter Xu a écrit :
>>>> On Fri, Nov 24, 2023 at 11:07:51AM -0500, Peter Xu wrote:
>>>>> On Fri, Nov 24, 2023 at 09:06:01AM +0000, Ryan Roberts wrote:
>>>>>> I don't have any micro-benchmarks for GUP though, if that's your question. Is
>>>>>> there an easy-to-use test I can run to get some numbers? I'd be happy to try it out.
>>>>>
>>>>> Thanks Ryan.  Then nothing is needed to be tested if gup is not yet touched
>>>>> from your side, afaict.  I'll see whether I can provide some rough numbers
>>>>> instead in the next post (I'll probably only be able to test it in a VM,
>>>>> though, but hopefully that should still reflect mostly the truth).
>>>>
>>>> An update: I finished a round of 64K cont_pte test, in the slow gup micro
>>>> benchmark I see ~15% perf degrade with this patchset applied on a VM on top
>>>> of Apple M1.
>>>>
>>>> Frankly that's even less than I expected, considering not only how slow gup
>>>> THP used to be, but also on the fact that that's a tight loop over slow
>>>> gup, which in normal cases shouldn't happen: "present" ptes normally goes
>>>> to fast-gup, while !present goes into a fault following it.  I assume
>>>> that's why nobody cared slow gup for THP before.  I think adding cont_pte
>>>> support shouldn't be very hard, but that will include making cont_pte idea
>>>> global just for arm64 and riscv Svnapot.
>>>
>>> Is there any documentation on what cont_pte is ? I have always wondered
>>> if it could also fit powerpc 8xx need ?
>>
>> pte_cont() (and pte_mkcont() and pte_mknoncont()) test and manipulte the
>> "contiguous bit" in the arm64 PTE entries. Those helpers are arm64-specific
>> (AFAIK). The contiguous bit is a hint to the HW to tell it that a block of PTEs
>> are mapping a physically contiguous and naturally aligned piece of memory. The
>> HW can use this to coalesce entries in the TLB. When using 4K base pages, the
>> contpte size is 64K (16 PTEs). For 16K base pages, its 2M (128 PTEs) and for 64K
>> base pages, its 2M (32 PTEs).
>>
>>>
>>> On powerpc, for 16k pages, we have to define 4 consecutive PTEs. All 4
>>> PTE are flagged with the SPS bit telling it's a 16k pages, but for TLB
>>> misses the HW needs one entrie for each 4k fragment.
>>
>>  From that description, it sounds like the SPS bit might be similar to arm64
>> contiguous bit? Although sounds like you are currently using it in a slightly
>> different way - telling kernel that the base page is 16K but mapping each 16K
>> page with 4x 4K entries (plus the SPS bit set)?
> 
> Yes it's both.
> 
> When the base page is 16k, there are 4x 4k entries (with SPS bit set) in 
> the page table, and pte_t is a table of 4 'unsigned long'
> 
> When the base page is 4k, there is a 16k hugepage size, which is the 
> same 4x 4k entries with SPS bit set.
> 
> So it looks similar to the contiguous bit.
> 
> 
> And by extension, the same principle is used for 512k hugepages, the bit 
> _PAGE_HUGE is copied by the TLB miss handler into the lower bit of PS, 
> PS being as follows:
> - 00 Small (4 Kbyte or 16 Kbyte)
> - 01 512 Kbyte
> - 10 Reserved
> - 11 8 Mbyte
> 
> So as PMD size is 4M, 512k pages are 128 identical consecutive entries 
> in the page table.
> 
> I which I could have THP with 16k or 512k pages.

Then you have come to the right place! :)

https://lore.kernel.org/linux-mm/20231204102027.57185-1-ryan.roberts@arm.com/


> 
> Christophe