linux-kernel - Re: [PATCH v3] mm/thp: fix "mm: thp: kill __transhuge_page

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Date:   Thu, 24 Aug 2023 07:47:03 -0700
From:   "Zach O'Keefe" <zokeefe@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, Yang Shi <shy828301@...il.com>,
        linux-kernel@...r.kernel.org,
        Saurabh Singh Sengar <ssengar@...rosoft.com>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v3] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

On Thu, Aug 24, 2023 at 7:05 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 24.08.23 15:59, Zach O'Keefe wrote:
> > On Thu, Aug 24, 2023 at 12:39 AM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> On 22.08.23 01:48, Zach O'Keefe wrote:
> >>> The 6.0 commits:
> >>>
> >>> commit 9fec51689ff6 ("mm: thp: kill transparent_hugepage_active()")
> >>> commit 7da4e2cb8b1f ("mm: thp: kill __transhuge_page_enabled()")
> >>>
> >>> merged "can we have THPs in this VMA?" logic that was previously done
> >>> separately by fault-path, khugepaged, and smaps "THPeligible" checks.
> >>>
> >>> During the process, the semantics of the fault path check changed in two
> >>> ways:
> >>>
> >>> 1) A VM_NO_KHUGEPAGED check was introduced (also added to smaps path).
> >>> 2) We no longer checked if non-anonymous memory had a vm_ops->huge_fault
> >>>      handler that could satisfy the fault.  Previously, this check had been
> >>>      done in create_huge_pud() and create_huge_pmd() routines, but after
> >>>      the changes, we never reach those routines.
> >>>
> >>> During the review of the above commits, it was determined that in-tree
> >>> users weren't affected by the change; most notably, since the only relevant
> >>> user (in terms of THP) of VM_MIXEDMAP or ->huge_fault is DAX, which is
> >>> explicitly approved early in approval logic.  However, there is at least
> >>> one occurrence where an out-of-tree driver that used
> >>> VM_HUGEPAGE|VM_MIXEDMAP with a vm_ops->huge_fault handler, was broken.
> >>
> >> ... so all we did is break an arbitrary out-of-tree driver? Sorry to
> >> say, but why should we care?
> >>
> >> Is there any in-tree code affected and needs a "Fixes:" ?
> >
> > The in-tree code was taken care of during the rework .. but I didn't
> > know about the possibility of a driver hooking in here.
>
> And that's the problem of the driver, no? It's not the job of the kernel
> developers to be aware of each out-of-tree driver to not accidentally
> break something in there.
>
> >
> > I don't know what the normal policy / stance here is, but I figured
> > the change was simple enough that it was worth helping out.
>
> If you decide to be out-of-tree, then you have be prepared to only
> support tested kernels and fix your driver when something changes
> upstream -- like upstream developers would do for you when it would be
> in-tree.
>
> So why can't the out-of-tree driver be fixed, similarly to how we would
> have fixed it if it would be in-tree?

I don't know much about driver development, but perhaps they are /
need to use a pristine upstream kernel, with their driver as a
loadable kernel module. Saurabh can comment on this, I don't know.

But your point is very valid otherwise.


> --
> Cheers,
>
> David / dhildenb
>