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

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Date: Mon, 1 Jul 2024 12:22:09 +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 12:16, Ryan Roberts wrote:
> On 01/07/2024 10:17, David Hildenbrand wrote:
>> 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]
> 
> Hmm... baby brain?

:)

I think I only mentioned it in a private mail at some point.

> 
> Sorry about that. I've been a bit useless lately. For some reason it wasn't on
> my list, but its there now. Will prioritise it, because I agree it's not good.


IIRC, if you do

echo never > /sys/kernel/mm/transparent_hugepage/enabled
echo always > /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled

khugepaged will not get activated.

-- 
Cheers,

David / dhildenb