Re: [PATCH v2 4/4] mm: split underutilized THPs

[Usama Arif <usamaarif642@gmail.com>]

On 09/08/2024 14:21, David Hildenbrand wrote:
> On 09.08.24 12:31, Usama Arif wrote:
>>
>>
>> On 08/08/2024 16:55, David Hildenbrand wrote:
>>> On 07.08.24 15:46, Usama Arif wrote:
>>>> This is an attempt to mitigate the issue of running out of memory when THP
>>>> is always enabled. During runtime whenever a THP is being faulted in
>>>> (__do_huge_pmd_anonymous_page) or collapsed by khugepaged
>>>> (collapse_huge_page), the THP is added to  _deferred_list. Whenever memory
>>>> reclaim happens in linux, the kernel runs the deferred_split
>>>> shrinker which goes through the _deferred_list.
>>>>
>>>> If the folio was partially mapped, the shrinker attempts to split it.
>>>> A new boolean is added to be able to distinguish between partially
>>>> mapped folios and others in the deferred_list at split time in
>>>> deferred_split_scan. Its needed as __folio_remove_rmap decrements
>>>> the folio mapcount elements, hence it won't be possible to distinguish
>>>> between partially mapped folios and others in deferred_split_scan
>>>> without the boolean.
>>>
>>> Just so I get this right: Are you saying that we might now add fully mapped folios to the deferred split queue and that's what you want to distinguish?
>>
>> Yes
>>
>>>
>>> If that's the case, then could we use a bit in folio->_flags_1 instead?
>> Yes, thats a good idea. Will create the below flag for the next revision
>>
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index 5769fe6e4950..5825bd1cf6db 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
>> @@ -189,6 +189,11 @@ enum pageflags {
>>     #define PAGEFLAGS_MASK         ((1UL << NR_PAGEFLAGS) - 1)
>>   +enum folioflags_1 {
>> +       /* The first 8 bits of folio->_flags_1 are used to keep track of folio order */
>> +       FOLIO_PARTIALLY_MAPPED = 8,     /* folio is partially mapped */
>> +}
> 
> This might be what you want to achieve:
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index a0a29bd092f8..d4722ed60ef8 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 */
>  };
>  
>  #define PAGEFLAGS_MASK         ((1UL << NR_PAGEFLAGS) - 1)
> @@ -861,8 +862,9 @@ static inline void ClearPageCompound(struct page *page)
>         ClearPageHead(page);
>  }
>  FOLIO_FLAG(large_rmappable, FOLIO_SECOND_PAGE)
> +FOLIO_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
>  #else
> -FOLIO_FLAG_FALSE(large_rmappable)
> +FOLIO_FLAG_FALSE(partially_mapped)
>  #endif
>  
>  #define PG_head_mask ((1UL << PG_head))
> 
> The downside is an atomic op to set/clear, but it should likely not really matter
> (initially, the flag will be clear, and we should only ever set it once when
> partially unmapping). If it hurts, we can reconsider.
> 
> [...]

I was looking for where the bits for flags_1 were specified! I just saw the start of enum pageflags, saw that compound order isn't specified anywhere over there and ignored the end :)

Yes, this is what I wanted to do. Thanks.