On Wed, Jun 29, 2022 at 7:01 AM Zach O'Keefe <zokeefe@xxxxxxxxxx> wrote: > > Hey All, > > There are currently a number of paths where we can collapse file memory into > THPs: > > 1a) khugepaged - target of vma being processed > 1b) khugepaged - other vma found mapping file, able to lock mmap_lock in > retract_page_tables() > 1b) khugepaged - other vma found mapping file, deferred pte-mapped THP collapse, > processed in collapse_pte_mapped_thp() > 2) page fault finds hugepage in page cache + filemap_map_pages() > > In terms of system-enforced THP constraints: > > * vma flags + thps sysfs settings > > Checked in 1a. (1b now at least respects "never" THP mode after Yang Shi's > cleanup series, but still doesn't respect "madvise" THP mode) > > * MMF_DISABLE_THP > > Checked in 1a and 1b > > I'm wondering if we should align these, and if so, in what direction? I would > argue that a process marked MMF_DISABLE_THP, or a vma marked VM_NOHUGEPAGE, > probably shouldn't be mapping at the pmd level, and that the appropriate checks > should be added in those paths. That depends on how we explain the semantics of MMF_DISABLE_THP and VM_NOHUGEPAGE IMHO. They definitely prevent from allocating/collapsing new THPs for that process or vma, but shall they also prevent from mapping existing THPs to PMD? Maybe not. > > Thanks, > Zach
