Re: [PATCH v1] mm: shmem: Rename mTHP shmem counters

[Ryan Roberts <ryan.roberts@arm.com>]

On 09/07/2024 09:13, Barry Song wrote:
> On Tue, Jul 9, 2024 at 7:55 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 09/07/2024 02:44, Barry Song wrote:
>>> On Tue, Jul 9, 2024 at 12:30 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>
>>>> On 08/07/2024 12:36, Barry Song wrote:
>>>>> On Mon, Jul 8, 2024 at 11:24 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>>>
>>>>>> The legacy PMD-sized THP counters at /proc/vmstat include
>>>>>> thp_file_alloc, thp_file_fallback and thp_file_fallback_charge, which
>>>>>> rather confusingly refer to shmem THP and do not include any other types
>>>>>> of file pages. This is inconsistent since in most other places in the
>>>>>> kernel, THP counters are explicitly separated for anon, shmem and file
>>>>>> flavours. However, we are stuck with it since it constitutes a user ABI.
>>>>>>
>>>>>> Recently, commit 66f44583f9b6 ("mm: shmem: add mTHP counters for
>>>>>> anonymous shmem") added equivalent mTHP stats for shmem, keeping the
>>>>>> same "file_" prefix in the names. But in future, we may want to add
>>>>>> extra stats to cover actual file pages, at which point, it would all
>>>>>> become very confusing.
>>>>>>
>>>>>> So let's take the opportunity to rename these new counters "shmem_"
>>>>>> before the change makes it upstream and the ABI becomes immutable.
>>>>>
>>>>> Personally, I think this approach is much clearer. However, I recall
>>>>> we discussed this
>>>>> before [1], and it seems that inconsistency is a concern?
>>>>
>>>> Embarrassingly, I don't recall that converstation at all :-| but at least what I
>>>> said then is consistent with what I've done in this patch.
>>>>
>>>> I think David's conclusion from that thread was to call them FILE_, and add both
>>>> shmem and pagecache counts to those counters, meaning we can keep the same name
>>>> as legacy THP counters. But those legacy THP counters only count shmem, and I
>>>> don't think we would get away with adding pagecache counts to those at this
>>>> point? (argument: they have been around for long time and there is a risk that
>>>> user space relies on them and if they were to dramatically increase due to
>>>> pagecache addition now that could break things). In that case, there is still
>>>> inconsistency, but its worse; the names are consistent but the semantics are
>>>> inconsistent.
>>>>
>>>> So my vote is to change to SHMEM_ as per this patch :)
>>>
>>> I have no objections. However, I dislike the documentation for
>>> thp_file_*. Perhaps we can clean it all up together ?
>>
>> I agree that we should clean this documentation up and I'm happy to roll it into
>> v2. However, I don't think what you have suggested is quite right.
>>
>> thp_file_alloc, thp_file_fallback and thp_file_fallback_charge *only* count
>> shmem. They don't count pagecache. So perhaps the change should be "...every
>> time a shmem huge page (dispite being named after "file", the counter measures
>> only shmem) is..."?
> 
> I understand what you are saying, and I know that thp_file_* has only
> included shmem so far. My question is whether it will include regular
> files in the future? If not, I am perfectly fine with your approach.

My whole reasoning for this patch is based on the assertion that since
THP_FILE_ALLOC has been there for 8 years and in all that time has only counted
shmem, then its highly likely that someone is depending on that semantic and we
can't change it. I don't have any actual evidence of code that relies on it though.

I propose I change the docs to reflect what's actually happening today (i.e.
shmem *only*). If we later decide we want to also report page cache numbers
through that same counter, then we can change the docs at that point. But if I
get my way, we'll soon have mTHP counters for FILE, which is solely for page
cache. So You'll be able to get all the fine-grained info out of those and there
will be no need to mess with the legacy counters.

> 
> READ_ONLY_THP_FOR_FS isn't applicable in this path as it is created
> by khugepaged collapse.
> 
>>
>> thp_file_mapped includes both file and shmem, so agree with your change there.
>>
>> What do you think?
>>
>>
>>>
>>> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
>>> index 709fe10b60f4..65df48cb3bbb 100644
>>> --- a/Documentation/admin-guide/mm/transhuge.rst
>>> +++ b/Documentation/admin-guide/mm/transhuge.rst
>>> @@ -417,21 +417,22 @@ thp_collapse_alloc_failed
>>>       the allocation.
>>>
>>>  thp_file_alloc
>>> -     is incremented every time a file huge page is successfully
>>> -     allocated.
>>> +     is incremented every time a file (including shmem) huge page is
>>> +     successfully allocated.
>>>
>>>  thp_file_fallback
>>> -     is incremented if a file huge page is attempted to be allocated
>>> -     but fails and instead falls back to using small pages.
>>> +     is incremented if a file (including shmem) huge page is attempted
>>> +     to be allocated but fails and instead falls back to using small
>>> +     pages.
>>>
>>>  thp_file_fallback_charge
>>> -     is incremented if a file huge page cannot be charged and instead
>>> -     falls back to using small pages even though the allocation was
>>> -     successful.
>>> +     is incremented if a file (including shmem) huge page cannot be
>>> +     charged and instead falls back to using small pages even though
>>> +     the allocation was successful.
>>>
>>>  thp_file_mapped
>>> -     is incremented every time a file huge page is mapped into
>>> -     user address space.
>>> +     is incremented every time a file (including shmem) huge page is
>>> +     mapped into user address space.
>>>
>>>  thp_split_page
>>>       is incremented every time a huge page is split into base
>>>
>>>>
>>>>>
>>>>> [1] https://lore.kernel.org/linux-mm/05d0096e4ec3e572d1d52d33a31a661321ac1551.1713755580.git.baolin.wang@linux.alibaba.com/
>>>>>
>>>>>
>>>>>>
>>>>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>>>>>> ---
>>>>>>
>>>>>> Hi All,
>>>>>>
>>>>>> Applies on top of today's mm-unstable (2073cda629a4) and tested with mm
>>>>>> selftests; no regressions observed.
>>>>>>
>>>>>> The backstory here is that I'd like to introduce some counters for regular file
>>>>>> folio allocations to observe how often large folio allocation succeeds, but
>>>>>> these shmem counters are named "file" which is going to make things confusing.
>>>>>> So hoping to solve that before commit 66f44583f9b6 ("mm: shmem: add mTHP
>>>>>> counters for anonymous shmem") goes upstream (it is currently in mm-stable).
>>>>>>
>>>>>> Admittedly, this change means the mTHP stat names are not the same as the legacy
>>>>>> PMD-size THP names, but I think that's a smaller issue than having "file_" mTHP
>>>>>> stats that only count shmem, then having to introduce "file2_" or "pgcache_"
>>>>>> stats for the regular file memory, which is even more inconsistent IMHO. I guess
>>>>>> the alternative is to count both shmem and file in these mTHP stats (that's how
>>>>>> they were documented anyway) but I think it's better to be able to consider them
>>>>>> separately like we do for all the other counters.
>>>>>>
>>>>>> Thanks,
>>>>>> Ryan
>>>>>>
>>>>>>  Documentation/admin-guide/mm/transhuge.rst | 12 ++++++------
>>>>>>  include/linux/huge_mm.h                    |  6 +++---
>>>>>>  mm/huge_memory.c                           | 12 ++++++------
>>>>>>  mm/shmem.c                                 |  8 ++++----
>>>>>>  4 files changed, 19 insertions(+), 19 deletions(-)
>>>>>>
>>>>>> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
>>>>>> index 747c811ee8f1..8b891689fc13 100644
>>>>>> --- a/Documentation/admin-guide/mm/transhuge.rst
>>>>>> +++ b/Documentation/admin-guide/mm/transhuge.rst
>>>>>> @@ -496,16 +496,16 @@ swpout_fallback
>>>>>>         Usually because failed to allocate some continuous swap space
>>>>>>         for the huge page.
>>>>>>
>>>>>> -file_alloc
>>>>>> -       is incremented every time a file huge page is successfully
>>>>>> +shmem_alloc
>>>>>> +       is incremented every time a shmem huge page is successfully
>>>>>>         allocated.
>>>>>>
>>>>>> -file_fallback
>>>>>> -       is incremented if a file huge page is attempted to be allocated
>>>>>> +shmem_fallback
>>>>>> +       is incremented if a shmem huge page is attempted to be allocated
>>>>>>         but fails and instead falls back to using small pages.
>>>>>>
>>>>>> -file_fallback_charge
>>>>>> -       is incremented if a file huge page cannot be charged and instead
>>>>>> +shmem_fallback_charge
>>>>>> +       is incremented if a shmem huge page cannot be charged and instead
>>>>>>         falls back to using small pages even though the allocation was
>>>>>>         successful.
>>>>>>
>>>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>>>>> index acb6ac24a07e..cff002be83eb 100644
>>>>>> --- a/include/linux/huge_mm.h
>>>>>> +++ b/include/linux/huge_mm.h
>>>>>> @@ -269,9 +269,9 @@ enum mthp_stat_item {
>>>>>>         MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
>>>>>>         MTHP_STAT_SWPOUT,
>>>>>>         MTHP_STAT_SWPOUT_FALLBACK,
>>>>>> -       MTHP_STAT_FILE_ALLOC,
>>>>>> -       MTHP_STAT_FILE_FALLBACK,
>>>>>> -       MTHP_STAT_FILE_FALLBACK_CHARGE,
>>>>>> +       MTHP_STAT_SHMEM_ALLOC,
>>>>>> +       MTHP_STAT_SHMEM_FALLBACK,
>>>>>> +       MTHP_STAT_SHMEM_FALLBACK_CHARGE,
>>>>>>         MTHP_STAT_SPLIT,
>>>>>>         MTHP_STAT_SPLIT_FAILED,
>>>>>>         MTHP_STAT_SPLIT_DEFERRED,
>>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>>> index 9ec64aa2be94..f9696c94e211 100644
>>>>>> --- a/mm/huge_memory.c
>>>>>> +++ b/mm/huge_memory.c
>>>>>> @@ -568,9 +568,9 @@ DEFINE_MTHP_STAT_ATTR(anon_fault_fallback, MTHP_STAT_ANON_FAULT_FALLBACK);
>>>>>>  DEFINE_MTHP_STAT_ATTR(anon_fault_fallback_charge, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
>>>>>>  DEFINE_MTHP_STAT_ATTR(swpout, MTHP_STAT_SWPOUT);
>>>>>>  DEFINE_MTHP_STAT_ATTR(swpout_fallback, MTHP_STAT_SWPOUT_FALLBACK);
>>>>>> -DEFINE_MTHP_STAT_ATTR(file_alloc, MTHP_STAT_FILE_ALLOC);
>>>>>> -DEFINE_MTHP_STAT_ATTR(file_fallback, MTHP_STAT_FILE_FALLBACK);
>>>>>> -DEFINE_MTHP_STAT_ATTR(file_fallback_charge, MTHP_STAT_FILE_FALLBACK_CHARGE);
>>>>>> +DEFINE_MTHP_STAT_ATTR(shmem_alloc, MTHP_STAT_SHMEM_ALLOC);
>>>>>> +DEFINE_MTHP_STAT_ATTR(shmem_fallback, MTHP_STAT_SHMEM_FALLBACK);
>>>>>> +DEFINE_MTHP_STAT_ATTR(shmem_fallback_charge, MTHP_STAT_SHMEM_FALLBACK_CHARGE);
>>>>>>  DEFINE_MTHP_STAT_ATTR(split, MTHP_STAT_SPLIT);
>>>>>>  DEFINE_MTHP_STAT_ATTR(split_failed, MTHP_STAT_SPLIT_FAILED);
>>>>>>  DEFINE_MTHP_STAT_ATTR(split_deferred, MTHP_STAT_SPLIT_DEFERRED);
>>>>>> @@ -581,9 +581,9 @@ static struct attribute *stats_attrs[] = {
>>>>>>         &anon_fault_fallback_charge_attr.attr,
>>>>>>         &swpout_attr.attr,
>>>>>>         &swpout_fallback_attr.attr,
>>>>>> -       &file_alloc_attr.attr,
>>>>>> -       &file_fallback_attr.attr,
>>>>>> -       &file_fallback_charge_attr.attr,
>>>>>> +       &shmem_alloc_attr.attr,
>>>>>> +       &shmem_fallback_attr.attr,
>>>>>> +       &shmem_fallback_charge_attr.attr,
>>>>>>         &split_attr.attr,
>>>>>>         &split_failed_attr.attr,
>>>>>>         &split_deferred_attr.attr,
>>>>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>>>>> index 921d59c3d669..f24dfbd387ba 100644
>>>>>> --- a/mm/shmem.c
>>>>>> +++ b/mm/shmem.c
>>>>>> @@ -1777,7 +1777,7 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
>>>>>>                         if (pages == HPAGE_PMD_NR)
>>>>>>                                 count_vm_event(THP_FILE_FALLBACK);
>>>>>>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>>>> -                       count_mthp_stat(order, MTHP_STAT_FILE_FALLBACK);
>>>>>> +                       count_mthp_stat(order, MTHP_STAT_SHMEM_FALLBACK);
>>>>>>  #endif
>>>>>>                         order = next_order(&suitable_orders, order);
>>>>>>                 }
>>>>>> @@ -1804,8 +1804,8 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
>>>>>>                                 count_vm_event(THP_FILE_FALLBACK_CHARGE);
>>>>>>                         }
>>>>>>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>>>> -                       count_mthp_stat(folio_order(folio), MTHP_STAT_FILE_FALLBACK);
>>>>>> -                       count_mthp_stat(folio_order(folio), MTHP_STAT_FILE_FALLBACK_CHARGE);
>>>>>> +                       count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_FALLBACK);
>>>>>> +                       count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_FALLBACK_CHARGE);
>>>>>>  #endif
>>>>>>                 }
>>>>>>                 goto unlock;
>>>>>> @@ -2181,7 +2181,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
>>>>>>                         if (folio_test_pmd_mappable(folio))
>>>>>>                                 count_vm_event(THP_FILE_ALLOC);
>>>>>>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>>>> -                       count_mthp_stat(folio_order(folio), MTHP_STAT_FILE_ALLOC);
>>>>>> +                       count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_ALLOC);
>>>>>>  #endif
>>>>>>                         goto alloced;
>>>>>>                 }
>>>>>> --
>>>>>> 2.43.0
>>>>>>
>>>>>
> 
> Thanks
> Barry