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

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Message-ID: <PUZP153MB06350A5DC9CCB8448C98E4EEBE1DA@PUZP153MB0635.APCP153.PROD.OUTLOOK.COM>
Date:   Thu, 24 Aug 2023 15:39:02 +0000
From:   Saurabh Singh Sengar <ssengar@...rosoft.com>
To:     Zach O'Keefe <zokeefe@...gle.com>,
        David Hildenbrand <david@...hat.com>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Yang Shi <shy828301@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Saurabh Sengar <ssengar@...ux.microsoft.com>
Subject: RE: [EXTERNAL] 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.

You are correct Zach. The "out-of-tree" driver had been seamlessly operational
on version 5.19, leveraging the kernel's capability to handle huge faults for
non-anonymous vma. However, the transition to kernel version 6.1 inadvertently
led to the removal of this feature. It's important to note that this removal wasn't
intentional, and I am optimistic about the potential restoration of this feature.

Hello David,

Given the context, I am currently exploring potential ways to address the issue
with the "out-of-tree" driver. I have recognized a challenge posed by the kernel's
memory management framework, which now imposes restrictions on huge faults
for non-anonymous vma. My inclination to contribute this driver to the mainline
kernel remains strong. If there's a possibility of engaging in discussions or
collaborative efforts to align this driver with the present framework, I'm fully
committed to the process. Your suggestions would be greatly appreciated, as I
am eager to ensure the compatibility of the "out-of-tree" driver with the kernel's
evolving framework.

- Saurabh

> 
> But your point is very valid otherwise.
> 
> 
> > --
> > Cheers,
> >
> > David / dhildenb
> >