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

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Date: Mon, 1 Jul 2024 10:14:33 +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 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 :)