On Wed, Aug 16, 2023 at 02:31:06PM -0700, Zach O'Keefe wrote: > On Mon, Aug 14, 2023 at 7:24 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > So if we find a large folio that is PMD mappable, and there's nothing > > at vmf->pmd, we install a PMD-sized mapping at that spot. If that > > fails, we install the preallocated PTE table at vmf->pmd and continue to > > trying set one or more PTEs to satisfy this page fault. > > Aha! I see. I did not expect ->fault() to have this logic, as I had > incorrectly thought (aka assumed) the pmd vs pte-mapping logic split > at create_huge_pmd(); i.e. do_huge_pmd_anonymous_page(), or > ->huge_fault(), or fallback to pte-mapping. It seems very weird to me > that hugepage_vma_check() "artificially" says "no" to file and shmem > along the fault path, so they can go and do their own thing in > ->fault(). Wow, hugepage_vma_check() is a very complicated function. I'm glad I ignored it! > IIUC then, there is a bug in smaps THPeligible code when > CONFIG_READ_ONLY_THP_FOR_FS is not set. Not obvious, but apparently > this config is (according to it's Kconfig desc) khugepaged-only, so it > should be fine for it to be disabled, yet allow > do_sync_mmap_readahead() to install a pmd for file-backed memory. > hugepage_vma_check() will need to be patched to fix this. I guess so ... > But I have a larger question for you: should we care about > /sys/kernel/mm/transparent_hugepage/enabled for file-fault? We > currently don't. Seems weird that we can transparently get a hugepage > when THP="never". Also, if THP="always", we might as well skip the > VM_HUGEPAGE check, and try the final pmd install (and save khugepaged > the trouble of attempting it later). I deliberately ignored the humungous complexity of the THP options. They're overgrown and make my brain hurt. Instead, large folios are adaptive; they observe the behaviour of the user program and choose based on history what to do. This is far superior to having a sysadmin tell us what to do!
