Re: [PATCH v2] mm/readahead: Fix large folio support in async readahead

[David Hildenbrand <david@redhat.com>]

On 13.11.24 05:19, Matthew Wilcox wrote:
> On Tue, Nov 12, 2024 at 04:19:07PM +0100, David Hildenbrand wrote:
>> Someone configured: "Don't readahead more than 128KiB"
> 
> Did they, though?  I have nothing but contempt for the thousands of
> parameters that we expect sysadmins to configure.  It's ridiculous and
> it needs to stop.  So, we listen to the program that has told us "We
> want 2MB pages" and not to the sysadmin who hasn't changed the value of
> readahead from one that was originally intended for floppy discs.

If something can be achieved using MADV_HUGEPAGE but not using 
auto-tuning (ordinary readahead / no MADV_HUGEPAGE) it's a warning sign 
at least to me ...

FWIW, I agree that the parameters we have are confusing. But selectively 
ignoring them ... I don't know.

If the parameter is effectively useless on some devices (except floppy 
discs?) maybe it should be set to "0=auto" (and modification attempts 
failing/being ignored) or simply ignore for all of readahead.

Anyhow, your call. I'll see if I can make sense of the code and come up 
with a comment that explains the situation.

-- 
Cheers,

David / dhildenb