Re: [EXTERNAL] Re: [PATCH v3] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

[Saurabh Singh Sengar <ssengar@linux.microsoft.com>]

On Fri, Aug 25, 2023 at 08:09:07AM -0700, Zach O'Keefe wrote:
> On Fri, Aug 25, 2023 at 5:58 AM David Hildenbrand <david@redhat.com> wrote:
> >
> > On 25.08.23 14:49, Matthew Wilcox wrote:
> > > On Fri, Aug 25, 2023 at 09:59:23AM +0200, David Hildenbrand wrote:
> > >> Especially, we do have bigger ->huge_fault changes coming up:
> > >>
> > >> https://lkml.kernel.org/r/20230818202335.2739663-1-willy@infradead.org
> 
> FWIW, one of those patches updates the docs to read,
> 
> "->huge_fault() is called when there is no PUD or PMD entry present.  This
> gives the filesystem the opportunity to install a PUD or PMD sized page.
> Filesystems can also use the ->fault method to return a PMD sized page,
> so implementing this function may not be necessary.  In particular,
> filesystems should not call filemap_fault() from ->huge_fault(). [..]"
> 
> Which won't work (in the general case) without this patch (well, at
> least the ->huge_fault() check part).
> 
> So, if we're advertising this is the way it works, maybe that gives a
> stronger argument for addressing it sooner vs when the first in-tree
> user depends on it?
> 
> > >> If the driver is not in the tree, people don't care.
> > >>
> > >> You really should try upstreaming that driver.
> > >>
> > >>
> > >> So this patch here adds complexity (which I don't like) in order to keep an
> > >> OOT driver working -- possibly for a short time. I'm tempted to say "please
> > >> fix your driver to not use huge faults in that scenario, it is no longer
> > >> supported".
> > >>
> > >> But I'm just about to vanish for 1.5 week into vacation :)
> > >>
> > >> @Willy, what are your thoughts?
> > >
> > > Fundamentally there was a bad assumption with the original patch --
> > > it assumed that the only reason to support ->huge_fault was for DAX,
> > > and that's not true.  It's just that the only drivers in-tree which
> > > support ->huge_fault do so in order to support DAX.
> >
> > Okay, and we are willing to continue supporting that then and it's
> > nothing we want to stop OOT drivers from doing.
> >
> > Fine with me; we should probably reflect that in the patch description.
> 
> I can change these paragraphs,
> 
> "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.
> 
> Remove the VM_NO_KHUGEPAGED check when not in collapse path and give
> any ->huge_fault handler a chance to handle the fault.  Note that we
> don't validate the file mode or mapping alignment, which is consistent
> with the behavior before the aforementioned commits."
> 
> To read,
> 
> "The above commits, however, overfit the existing in-tree use cases,
> and assume that
> the only reason to support ->huge_fault was for DAX (which is
> explicitly approved early in the approval logic).
> This is a bad assumption to make and unnecessarily prevents general
> support of ->huge_fault by filesystems. Allow returning "true" if such
> a handler exists, giving the fault path an opportunity to exercise it.
> 
> Similarly, the rationale for including the VM_NO_KHUGEPAGED check
> along the fault path was that it didn't alter any in-tree users, but
> was likewise similarly unnecessarily restrictive (and reads odd).
> Remove the check from the fault path."
>


Any chance this can make it to 6.6 kernel ?

- Saurabh