Re: [PATCH v3] mm/filemap: Allow arch to request folio size for exec memory

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

On 28/03/2025 09:32, Zi Yan wrote:
> On 28 Mar 2025, at 9:09, Ryan Roberts wrote:
> 
>> On 27/03/2025 20:07, Zi Yan wrote:
>>> On 27 Mar 2025, at 12:44, Matthew Wilcox wrote:
>>>
>>>> On Thu, Mar 27, 2025 at 04:06:58PM +0000, Ryan Roberts wrote:
>>>>> So let's special-case the read(ahead) logic for executable mappings. The
>>>>> trade-off is performance improvement (due to more efficient storage of
>>>>> the translations in iTLB) vs potential read amplification (due to
>>>>> reading too much data around the fault which won't be used), and the
>>>>> latter is independent of base page size. I've chosen 64K folio size for
>>>>> arm64 which benefits both the 4K and 16K base page size configs and
>>>>> shouldn't lead to any read amplification in practice since the old
>>>>> read-around path was (usually) reading blocks of 128K. I don't
>>>>> anticipate any write amplification because text is always RO.
>>>>
>>>> Is there not also the potential for wasted memory due to ELF alignment?
>>>> Kalesh talked about it in the MM BOF at the same time that Ted and I
>>>> were discussing it in the FS BOF.  Some coordination required (like
>>>> maybe Kalesh could have mentioned it to me rathere than assuming I'd be
>>>> there?)
>>>>
>>>>> +#define arch_exec_folio_order() ilog2(SZ_64K >> PAGE_SHIFT)
>>>>
>>>> I don't think the "arch" really adds much value here.
>>>>
>>>> #define exec_folio_order()	get_order(SZ_64K)
>>>
>>> How about AMD’s PTE coalescing, which does PTE compression at
>>> 16KB or 32KB level? It covers 4 16KB and 2 32KB, at least it will
>>> not hurt AMD PTE coalescing. Starting with 64KB across all arch
>>> might be simpler to see the performance impact. Just a comment,
>>> no objection. :)
>>
>> exec_folio_order() is defined per-architecture and SZ_64K is the arm64 preferred
>> size. At the moment x86 is not opted in, but they could choose to opt in with
>> 32K (or whatever else makese sense) if the HW supports coalescing.
> 
> Oh, I missed that part. I thought, since arch_ is not there, it was the same
> for all arch.
> >>
>> I'm not sure if you thought this was global and are arguing against that, or if
>> you are arguing for it to be global because it will more easily show us
>> performance regressions earlier if x86 is doing this too?
> 
> I thought it was global. It might be OK to set it global and let different arch
> to optimize it as it rolls out. Opt-in might be "never" until someone looks
> into it, but if it is global and it changes performance, people will notice
> and look into it.

Ahh now that we are both clear, I'd prefer to stick with the policy as
implemented; exec_folio_order() defaults to "use the existing readahead method"
but can be overridden by arches (arm64) that want specific behaviour (64K folios).

> 
> --
> Best Regards,
> Yan, Zi