Date: Mon, 1 Jul 2024 11:20:27 -0700
From: Yang Shi <shy828301@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: 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, 
	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 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. But the above setting
sounds confusing, can we disable the top level knob, but enable it on
a per-order basis? TBH, it sounds weird and doesn't make too much
sense to me.

>
> --
> Cheers,
>
> David / dhildenb
>
>

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