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... 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.