linux-kernel - Re: [PATCH] support "THPeligible" semantics for mTHP with anonymous shmem

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Date: Mon, 1 Jul 2024 11:17:03 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Bang Li
 <libang.li@...group.com>, hughd@...gle.com, akpm@...ux-foundation.org
Cc: wangkefeng.wang@...wei.com, ziy@...dia.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH] support "THPeligible" semantics for mTHP with anonymous
 shmem

On 01.07.24 11:14, Ryan Roberts wrote:
> On 01/07/2024 09:57, David Hildenbrand wrote:
>> On 01.07.24 10:50, Ryan Roberts wrote:
>>> On 01/07/2024 09:48, David Hildenbrand wrote:
>>>> On 01.07.24 10:40, Ryan Roberts wrote:
>>>>> On 01/07/2024 09:33, Baolin Wang wrote:
>>>>>>
>>>>>>
>>>>>> On 2024/7/1 15:55, Ryan Roberts wrote:
>>>>>>> On 28/06/2024 11:49, Bang Li wrote:
>>>>>>>> After the commit 7fb1b252afb5 ("mm: shmem: add mTHP support for
>>>>>>>> anonymous shmem"), we can configure different policies through
>>>>>>>> the multi-size THP sysfs interface for anonymous shmem. But
>>>>>>>> currently "THPeligible" indicates only whether the mapping is
>>>>>>>> eligible for allocating THP-pages as well as the THP is PMD
>>>>>>>> mappable or not for anonymous shmem, we need to support semantics
>>>>>>>> for mTHP with anonymous shmem similar to those for mTHP with
>>>>>>>> anonymous memory.
>>>>>>>>
>>>>>>>> Signed-off-by: Bang Li <libang.li@...group.com>
>>>>>>>> ---
>>>>>>>>      fs/proc/task_mmu.c      | 10 +++++++---
>>>>>>>>      include/linux/huge_mm.h | 11 +++++++++++
>>>>>>>>      mm/shmem.c              |  9 +--------
>>>>>>>>      3 files changed, 19 insertions(+), 11 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>>>>>>>> index 93fb2c61b154..09b5db356886 100644
>>>>>>>> --- a/fs/proc/task_mmu.c
>>>>>>>> +++ b/fs/proc/task_mmu.c
>>>>>>>> @@ -870,6 +870,7 @@ static int show_smap(struct seq_file *m, void *v)
>>>>>>>>      {
>>>>>>>>          struct vm_area_struct *vma = v;
>>>>>>>>          struct mem_size_stats mss = {};
>>>>>>>> +    bool thp_eligible;
>>>>>>>>            smap_gather_stats(vma, &mss, 0);
>>>>>>>>      @@ -882,9 +883,12 @@ static int show_smap(struct seq_file *m, void *v)
>>>>>>>>            __show_smap(m, &mss, false);
>>>>>>>>      -    seq_printf(m, "THPeligible:    %8u\n",
>>>>>>>> -           !!thp_vma_allowable_orders(vma, vma->vm_flags,
>>>>>>>> -               TVA_SMAPS | TVA_ENFORCE_SYSFS, THP_ORDERS_ALL));
>>>>>>>> +    thp_eligible = !!thp_vma_allowable_orders(vma, vma->vm_flags,
>>>>>>>> +                        TVA_SMAPS | TVA_ENFORCE_SYSFS, THP_ORDERS_ALL);
>>>>>>>> +    if (vma_is_anon_shmem(vma))
>>>>>>>> +        thp_eligible =
>>>>>>>> !!shmem_allowable_huge_orders(file_inode(vma->vm_file),
>>>>>>>> +                            vma, vma->vm_pgoff, thp_eligible);
>>>>>>>
>>>>>>> Afraid I haven't been following the shmem mTHP support work as much as I
>>>>>>> would
>>>>>>> have liked, but is there a reason why we need a separate function for shmem?
>>>>>>
>>>>>> Since shmem_allowable_huge_orders() only uses shmem specific logic to
>>>>>> determine
>>>>>> if huge orders are allowable, there is no need to complicate the
>>>>>> thp_vma_allowable_orders() function by adding more shmem related logic, making
>>>>>> it more bloated. In my view, providing a dedicated helper
>>>>>> shmem_allowable_huge_orders(), specifically for shmem, simplifies the logic.
>>>>>
>>>>> My point was really that a single interface (thp_vma_allowable_orders)
>>>>> should be
>>>>> used to get this information. I have no strong opinon on how the implementation
>>>>> of that interface looks. What you suggest below seems perfectly reasonable
>>>>> to me.
>>>>
>>>> Right. thp_vma_allowable_orders() might require some care as discussed in other
>>>> context (cleanly separate dax and shmem handling/orders). But that would be
>>>> follow-up cleanups.
>>>
>>> Are you planning to do that, or do you want me to send a patch?
>>
>> I'm planning on looking into some details, especially the interaction with large
>> folios in the pagecache. I'll let you know once I have a better idea what
>> actually should be done :)
> 
> OK great - I'll scrub it from my todo list... really getting things done today :)

Resolved the khugepaged thiny already? :P

[khugepaged not active when only enabling the sub-size via the 2M folder 
IIRC]

-- 
Cheers,

David / dhildenb