On Thu, Aug 17, 2023 at 10:47 AM Yang Shi <shy828301@xxxxxxxxx> wrote: > > On Wed, Aug 16, 2023 at 2:48 PM Zach O'Keefe <zokeefe@xxxxxxxxxx> wrote: > > > > > We have a out of tree driver that maps huge pages through a file handle and > > > relies on -> huge_fault. It used to work in 5.19 kernels but 6.1 changed this > > > behaviour. > > > > > > I don’t think reverting the earlier behaviour of fault_path for huge pages should > > > impact kernel negatively. > > > > > > Do you think we can restore this earlier behaviour of kernel to allow page fault > > > for huge pages via ->huge_fault. > > > > That seems reasonable to me. I think using the existence of a > > ->huge_fault() handler as a predicate to return "true" makes sense to > > me. The "normal" flow for file-backed memory along fault path still > > needs to return "false", so that we correctly fallback to ->fault() > > handler. Unless there are objections, I can do that in a v2. > > Sorry for chiming in late. I'm just back from vacation and trying to catch up... > > IIUC the out-of-tree driver tries to allocate huge page and install > PMD mapping via huge_fault() handler, but the cleanup of > hugepage_vma_check() prevents this due to the check to > VM_NO_KHUGEPAGED? > > So you would like to check whether a huge_fault() handler existed > instead of vma_is_dax()? Sorry for the multiple threads here. There are two problems: (a) the VM_NO_KHUGEPAGED check along fault path, and (b) we don't give ->huge_fault() a fair shake, if it exists, along fault path. The current code assumes vma_is_dax() iff ->huge_fault() exists. (a) is easy enough to fix. For (b), I'm currently looking at the possibility of not worrying about ->huge_fault() in hugepage_vma_check(), and just letting create_huge_pud() / create_huge_pmd() check and fallback as necessary. I think we'll need the explicit DAX check still, since we want to keep khugepaged and MADV_COLLAPSE away, and the presence / absence of ->huge_fault() isn't enough to know that (well.. today it kind of is, but we shouldn't depend on it).
