Re: [PATCH v1] mm: Fix khugepaged activation policy

[David Hildenbrand <david@redhat.com>]

On 02.07.24 17:29, Ryan Roberts wrote:
> On 02/07/2024 15:57, David Hildenbrand wrote:
>> On 02.07.24 16:46, Ryan Roberts wrote:
>>> Since the introduction of mTHP, the docuementation has stated that
>>> khugepaged would be enabled when any mTHP size is enabled, and disabled
>>> when all mTHP sizes are disabled. There are 2 problems with this; 1.
>>> this is not what was implemented by the code and 2. this is not the
>>> desirable behavior.
>>>
>>> Desirable behavior is for khugepaged to be enabled when any PMD-sized
>>> THP is enabled, anon or file. (Note that file THP is still controlled by
>>> the top-level control so we must always consider that, as well as the
>>> PMD-size mTHP control for anon). khugepaged only supports collapsing to
>>> PMD-sized THP so there is no value in enabling it when PMD-sized THP is
>>> disabled. So let's change the code and documentation to reflect this
>>> policy.
>>>
>>> Further, per-size enabled control modification events were not
>>> previously forwarded to khugepaged to give it an opportunity to start or
>>> stop. Consequently the following was resulting in khugepaged eroneously
>>> not being activated:
>>>
>>>     echo never > /sys/kernel/mm/transparent_hugepage/enabled
>>>     echo always > /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
>>>
>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>>> Fixes: 3485b88390b0 ("mm: thp: introduce multi-size THP sysfs interface")
>>> Closes:
>>> https://lore.kernel.org/linux-mm/7a0bbe69-1e3d-4263-b206-da007791a5c4@redhat.com/
>>> Cc: stable@vger.kernel.org
>>> ---
>>>
>>> Hi All,
>>>
>>> Applies on top of today's mm-unstable (9bb8753acdd8). No regressions observed in
>>> mm selftests.
>>>
>>> When fixing this I also noticed that khugepaged doesn't get (and never has been)
>>> activated/deactivated by `shmem_enabled=`. I'm not sure if khugepaged knows how
>>> to collapse shmem - perhaps it should be activated in this case?
>>>
>>
>> Call me confused.
>>
>> khugepaged_scan_mm_slot() and madvise_collapse() only all
>> hpage_collapse_scan_file() with ... IS_ENABLED(CONFIG_SHMEM) ?
> 
> Looks like khugepaged_scan_mm_slot() was converted from:
> 
>    if (shmem_file(vma->vm_file)) {
> 
> to:
> 
>    if (IS_ENABLED(CONFIG_SHMEM) && vma->vm_file) {
> 
> By 99cb0dbd47a15d395bf3faa78dc122bc5efe3fc0 which adds THP collapse support for
> non-shmem files. Clearly that looks wrong, but I guess never spotted in practice
> because noone disables shemem?
> 
> I guess madvise_collapse() was a copy/paste?
> 

Likely.

>>
>> collapse_file() is only called by hpage_collapse_scan_file() ... and there we
>> check "shmem_file(file)".
>>
>> So why is the IS_ENABLED(CONFIG_SHMEM) check in there if collapse_file() seems
>> to "collapse filemap/tmpfs/shmem pages into huge one".
>>
>> Anyhow, we certainly can collapse shmem (that's how it all started IIUC).
> 
> Yes, thanks for pointing me at it. Should have just searched "shmem" in
> khugepaged.c :-/
> 
>>
>> Besides that, khugepaged only seems to collapse !shmem with
>>    VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
> 
> That makes sense. I guess I could use IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) to
> tighen the (non-shmem) file THP check in hugepage_pmd_enabled() (currently I'm
> unconditionally using the top-level enabled setting as a "is THP enabled for
> files" check).
> 
> But back to my original question, I think hugepage_pmd_enabled() should also be
> explicitly checking the appropriate shmem_enabled controls and ORing in the
> result? Otherwise in a situation where only shmem is THP enabled (and file/anon
> THP is disabled) khugepaged won't run.

I think so.

> 
>>
>> The thp_vma_allowable_order() check tests if we are allowed to collapse a
>> PMD_ORDER in that VMA.
> 
> I don't follow the relevance of this statement.

On whatever VMA we indicate true, we will try to collapse in khugepaged. 
Regarding the question, what khugepaged will try to collapse.

-- 
Cheers,

David / dhildenb