Re: [PATCH v10 01/10] fs: Allow fine-grained control of folio sizes

Ryan Roberts
2024-07-17T15:26:20Z

On 17/07/2024 16:12, Pankaj Raghav (Samsung) wrote:
>>>>
>>>> This is really too much. It's something that will never happen. Just
>>>> delete the message.
>>>>
>>>>> + if (max > MAX_PAGECACHE_ORDER) {
>>>>> + VM_WARN_ONCE(1,
>>>>> + "max order > MAX_PAGECACHE_ORDER. Setting max_order to MAX_PAGECACHE_ORDER");
>>>>> + max = MAX_PAGECACHE_ORDER;
>>>>
>>>> Absolutely not. If the filesystem declares it can support a block size
>>>> of 4TB, then good for it. We just silently clamp it.
>>>
>>> Hmm, but you raised the point about clamping in the previous patches[1]
>>> after Ryan pointed out that we should not silently clamp the order.
>>>
>>> ```
>>>> It seems strange to silently clamp these? Presumably for the bs>ps usecase,
>>>> whatever values are passed in are a hard requirement? So wouldn't want them to
>>>> be silently reduced. (Especially given the recent change to reduce the size of
>>>> MAX_PAGECACHE_ORDER to less then PMD size in some cases).
>>>
>>> Hm, yes. We should probably make this return an errno. Including
>>> returning an errno for !IS_ENABLED() and min > 0.
>>> ```
>>>
>>> It was not clear from the conversation in the previous patches that we
>>> decided to just clamp the order (like it was done before).
>>>
>>> So let's just stick with how it was done before where we clamp the
>>> values if min and max > MAX_PAGECACHE_ORDER?
>>>
>>> [1] https://lore.kernel.org/linux-fsdevel/Zoa9rQbEUam467-q@casper.infradead.org/
>>
>> The way I see it, there are 2 approaches we could take:
>>
>> 1. Implement mapping_max_folio_size_supported(), write a headerdoc for
>> mapping_set_folio_order_range() that says min must be lte max, max must be lte
>> mapping_max_folio_size_supported(). Then emit VM_WARN() in
>> mapping_set_folio_order_range() if the constraints are violated, and clamp to
>> make it safe (from page cache's perspective). The VM_WARN()s can just be inline
>
> Inlining with the `if` is not possible since:
> 91241681c62a ("include/linux/mmdebug.h: make VM_WARN* non-rvals")

Ahh my bad. Could use WARN_ON()?

>
>> in the if statements to keep them clean. The FS is responsible for checking
>> mapping_max_folio_size_supported() and ensuring min and max meet requirements.
>
> This is sort of what is done here but IIUC willy's reply to the patch,
> he prefers silent clamping over having WARNINGS. I think because we check
> the constraints during the mount time, so it should be safe to call
> this I guess?

I don't want to put words in his mouth, but I thought he was complaining about
the verbosity of the warnings, not their presence.

>
>>
>> 2. Return an error from mapping_set_folio_order_range() (and the other functions
>> that set min/max). No need for warning. No state changed if error is returned.
>> FS can emit warning on error if it wants.
>
> I think Chinner was not happy with this approach because this is done
> per inode and basically we would just shutdown the filesystem in the
> first inode allocation instead of refusing the mount as we know about
> the MAX_PAGECACHE_ORDER even during the mount phase anyway.

Ahh that makes sense. Understood.

>
> --
> Pankaj