On Thu, Aug 17, 2023 at 5:18 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > 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! Ya it's a tangly area. Far better now though, then before Yang centralized everything. But yes, now I need to figure out what to do with 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 ... The easiest and most satisfying way to handle this -- and I think we talked about this before -- is relaxing that complicated file_thp_enabled() check when the file's mapping supports large folios. I think that makes sense to me, though I don't know all the details fs-side. Will we need any hook to give fs the chance to update any internal state on collapse? > > 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. [..] Same > [..] 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! I had written a bunch on this, but I arrived to the conclusion that (a) pmd-mapping here is ~ a free win, and (b) I'm not the best person to argue for these knobs, given MADV_COLLAPSE ignores them entirely :P ..But (sorry) what about MMF_DISABLE_THP?
