Message-ID: <0c5e16a6-fc23-4602-91d6-81a8742ba221@redhat.com>
Date: Tue, 2 Jul 2024 10:28:01 +0200
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, Yang Shi <shy828301@...il.com>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Bang Li <libang.li@...group.com>, hughd@...gle.com,
 akpm@...ux-foundation.org, 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 02.07.24 10:24, Ryan Roberts wrote:
> On 01/07/2024 19:20, Yang Shi wrote:
>> On Mon, Jul 1, 2024 at 3:23 AM David Hildenbrand <david@...hat.com> wrote:
>>>
>>> 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.
>>
>> khugepaged is controlled by the top level knob.
> 
> What do you mean by "top level knob"? I assume
> /sys/kernel/mm/transparent_hugepage/enabled ?
> 
> If so, that's not really a thing in its own right; its just the legacy PMD-size
> THP control, and we only take any notice of it if a per-size control is set to
> "inherit". So if we have:

In a simpler world, where "enabled" would have been a boolean (true / 
false), we could have made it a universal killswitch that is AND'ed with 
the other ones.

Unfortunately, we don't live in such a simple world.

-- 
Cheers,

David / dhildenb

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