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 mYsKGmIr 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> > On 2025/6/25 16:37, Lorenzo Stoakes wrote: > > Yeah maybe the best way is to just have another tunable for this? > > > > /sys/kernel/mm/transparent_hugepage/disable_collapse perhaps? > > > > What do you think Hugh, Baolin? > > I think it's not necessary to find a way to disable madvise_collapse. > Essentially, it's a conflict between the semantics of madvise_collapse and > the '/sys/kernel/mm/transparent_hugepage/enabled' interface. We should reach > a consensus on the semantics first: > > Semantic 1: madv_collapse() should ignore any THP system settings, meaning > we need to update the 'never' semantics in > '/sys/kernel/mm/transparent_hugepage/enabled', which would only disable page > fault and khugepaged, not including madvise_collapse. If we agree on this, > then the 'never' for per-sized mTHP would have the same semantics, i.e., > when I set 64K mTHP to 'always' and 2M mTHP to 'never', madvise_collapse > would still allow the collapse of 2M THP. We should document this clearly in > case users still want 64K mTHP from madvise_collapse. Right yeah, I mean this is in effect how things are now. So the task is documentation. > > > Semantic 2: madv_collapse() needs to respect THP system settings, which is > what my patch does. Never means never, and we would need to update the > documentation of madv_collapse() to make it clearer. Yes, and indeed this is the choice. I think, as David said, it comes down to whether we have a legit use case that truly relies on this. > > (One side note on PMD-sized MADV_COLLAPSE - this is basically completely > > useless for 64 KB page size arm64 systems where PMD's are 512 MB :) > > > > Thoughts Baolin? > > We should not collapse 512MB THP on 64K pagesize kernel. So seems > madv_collapse() can not work on 64K pagesize kernel. Well I don't think anything would prevent this now right? So MADV_COLLAPSE is pretty problematic on 64K pagesize kernels in general. Anyway that's maybe a problem for another time :) > > > > * If we drop the series, also consider how we might provide mTHP-compatible > > MADV_COLLAPSE. > > Yes. Agree. Will be another mess I guess :) Sigh yes sadly. Let's try and make it as unmessy as we can I guess :>)