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@xxxxxxx>
Fixes: 3485b88390b0 ("mm: thp: introduce multi-size THP sysfs interface")
Closes: https://lore.kernel.org/linux-mm/7a0bbe69-1e3d-4263-b206-da007791a5c4@xxxxxxxxxx/
Cc: stable@xxxxxxxxxxxxxxx
---
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
