[RFC PATCH v4 1/5] mm/readahead: Honour new_order in page_cache_ra_order()

[Ryan Roberts]

On 09/05/2025 21:50, Pankaj Raghav (Samsung) wrote:
>>>>  
>>>
>>> So we always had a fallback to do_page_cache_ra() if the size of the
>>> readahead is less than 4 pages (16k). I think this was there because we
>>> were adding `2` to the new_order:
>>
>> If this is the reason for the magic number 4, then it's a bug in itself IMHO. 4
>> pages is only 16K when the page size is 4K; arm64 supports other page sizes. But
>> additionally, it's not just ra->size that dictates the final order of the folio;
>> it also depends on alignment in the file, EOF, etc.
>>
> 
> IIRC, initially we were not able to use order-1 folios[1], so we always
> did a fallback for any order < 2 using do_page_cache_ra(). I think that
> is where the magic order 2 (4 pages) is coming. Please someone can
> correct me if I am wrong.

Ahh, I see. That might have been where it came from, but IMHO, it still didn't
really belong there; just because the size is bigger than 4 pages, it doesn't
mean you would never want to use order-1 folios - there are alignment
considerations that can cause that. The logic in page_cache_ra_order() used to
know to avoid order-1.

> 
> But we don't have that limitation for file-backed folios anymore, so the
> fallback for ra->size < 4 is probably not needed. So the only time we do
> a fallback is if we don't support large folios.
> 
>> If we remove the fallback condition completely, things will still work out. So
>> unless someone can explain the reason for that condition (Matthew?), my vote
>> would be to remove it entirely.
> 
> I am actually fine with removing the first part of this fallback condition.
> But as I said, we still need to do a fallback if we don't support large folios.

Yep agreed. I'll make this change in the next version.

> 
> --
> Pankaj
> 
> [1] https://lore.kernel.org/all/ZH0GvxAdw1RO2Shr@casper.infradead.org/