Re: [EXTERNAL] [PATCH] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

[Zach O'Keefe]

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).