linux-kernel - Re: [EXTERNAL] [PATCH] mm/thp: fix "mm: thp: kill __transhuge_page

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Date:   Thu, 17 Aug 2023 10:46:54 -0700
From:   Yang Shi <shy828301@...il.com>
To:     "Zach O'Keefe" <zokeefe@...gle.com>
Cc:     Saurabh Singh Sengar <ssengar@...rosoft.com>,
        Matthew Wilcox <willy@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [EXTERNAL] [PATCH] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

On Wed, Aug 16, 2023 at 2:48 PM Zach O'Keefe <zokeefe@...gle.com> 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()?