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i/nAFhqizlhIs0+qtdgs+uuhngQfXp1Mn9CWZBzzX9X22lU7jkCZyUzb9PPafVvClg+LUopUsqZ +MjQ9Rajhv1Srs7zFHYx3/wlm82zoqX/HoytWcqQqM35cBSR81RL/MYQkGuqi4urr0BLx9D3AZR EyGMkiamVlCyrj8X+432XzfAs8z9qRWU/Nq53GcixNN3pFAw2Slcjp7jmV5hHOn4B8dKIbK20lm OeozBsGYmdjqVQ+WLnpiDH2YP5PYthk1G1wxnIJvxQ3IW7ZkqyIbHVjZ39YGMwyXwZAizTENOT9 CVgLo/rQSuHwqlqdzzmi3N1HLxWUBtvh6JS0S2SAD2S5RCAe3oPx6Gdr2yBoFwWIBUQeL/40OYE Q2H9QlzG92C/GN8H3oLBEHK1toKU6ebfjlirDvItqSs0rM6p5pmyKnYg6amFahyvV9gxSl6D X-Authority-Analysis: v=2.4 cv=S5rZwJsP c=1 sm=1 tr=0 ts=685bc1fb b=1 cx=c_pps a=e1sVV491RgrpLwSTMOnk8w==:117 a=e1sVV491RgrpLwSTMOnk8w==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=kj9zAlcOel0A:10 a=6IFa9wvqVegA:10 a=GoEa3M9JfhUA:10 a=tcVnT6hyNcE7qv_QjisA:9 a=CjuIK1q_8ugA:10 cc=ntf awl=host:14714 On Wed, Jun 25, 2025 at 04:52:03PM +0800, Baolin Wang wrote: > > > 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 :>)