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

[David Hildenbrand <david@redhat.com>]

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) ?

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).

Besides that, khugepaged only seems to collapse !shmem with
   VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);

The thp_vma_allowable_order() check tests if we are allowed to collapse 
a PMD_ORDER in that VMA.

-- 
Cheers,

David / dhildenb