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

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Message-ID: <429f2873-8532-4cc8-b0e1-1c3de9f224d9@arm.com>
Date: Mon, 1 Jul 2024 11:16:19 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.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 01/07/2024 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?

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.