Hey Matthew, I'm leading an attempt to add a new madvise mode, MADV_COLLAPSE, to allow userspace-directed collapse of memory into THPs[1]. The initial proposal only supports anonymous memory, but I'm working on adding support for file-backed and shmem memory. The intended behavior of MADV_COLLAPSE is that it should return "success" if all hugepage-aligned / sized regions requested are backed by pmd-mapped THPs on return (races aside). IOW: we were able to successfully collapse the memory, or it was already backed by pmd-mapped THPs. Currently there is a nice "XXX: khugepaged should compact smaller compound pages into a PMD sized page" in khugepaged_scan_file() when we encounter a compound page during scanning. Do you know what kind of gotchas or technical difficulties would be involved in doing this? I presume this work would also benefit those relying on khugepaged to collapse read-only file and shmem memory, and I'd be happy to help move it forward. Thanks for your time, Zach [1] https://lore.kernel.org/linux-mm/20220504214437.2850685-1-zokeefe@xxxxxxxxxx/
