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

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

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..."?

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