Re: [PATCH v3 4/6] mm: Introduce a pageflag for partially mapped folios

[Usama Arif]

On 16/08/2024 17:28, Matthew Wilcox wrote:
> On Fri, Aug 16, 2024 at 05:08:35PM +0100, Usama Arif wrote:
>> On 16/08/2024 16:44, Matthew Wilcox wrote:
>>> On Tue, Aug 13, 2024 at 01:02:47PM +0100, Usama Arif wrote:
>>>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>>>> index a0a29bd092f8..cecc1bad7910 100644
>>>> --- a/include/linux/page-flags.h
>>>> +++ b/include/linux/page-flags.h
>>>> @@ -182,6 +182,7 @@ enum pageflags {
>>>>  	/* At least one page in this folio has the hwpoison flag set */
>>>>  	PG_has_hwpoisoned = PG_active,
>>>>  	PG_large_rmappable = PG_workingset, /* anon or file-backed */
>>>> +	PG_partially_mapped, /* was identified to be partially mapped */
>>>
>>> No, you can't do this.  You have to be really careful when reusing page
>>> flags, you can't just take the next one.  What made you think it would
>>> be this easy?
>>>
>>> I'd suggest using PG_reclaim.  You also need to add PG_partially_mapped
>>> to PAGE_FLAGS_SECOND.  You might get away without that if you're
>>> guaranteeing it'll always be clear when you free the folio; I don't
>>> understand this series so I don't know if that's true or not.
>>
>> I am really not sure what the issue is over here.
> 
> You've made the code more fragile.  It might happen to work today, but
> you've either done something which is subtly broken today, or might
> break tomorrow when somebody else rearranges the flags without knowing
> about your fragility.
> 
>> >From what I see, bits 0-7 of folio->_flags_1 are used for storing folio order, bit 8 for PG_has_hwpoisoned and bit 9 for PG_large_rmappable.
>> Bits 10 and above of folio->_flags_1 are not used any anywhere in the kernel. I am not reusing a page flag of folio->_flags_1, just taking an unused one.
> 
> No, wrong.  PG_anon_exclusive is used on every page, including tail
> pages, and that's above bit 10.
> 
>> Please have a look at the next few lines of the patch. I have defined the functions as FOLIO_FLAG(partially_mapped, FOLIO_SECOND_PAGE). I believe thats what you are saying in your second paragraph?
>> I am not sure what you meant by using PG_reclaim.
> 
> I mean:
> 
> -	PG_usama_new_thing,
> +	PG_usama_new_thing = PG_reclaim,
> 

Ah ok, Thanks. The flags below PG_reclaim are either marked as PF_ANY or are arch dependent. So eventhough they might not be used currently for _flags_1, they could be in the future.

I will use PG_partially_mapped = PG_reclaim in the next revision.

Thanks