Re: [PATCH v4] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 25 Sep 2023 13:01:10 -0700 "Zach O'Keefe" <zokeefe@google.com> 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, this was a bad
> assumption to make as it assumes the only reason to support ->huge_fault
> was for DAX (which is not true in general).
> 
> 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.

It's unclear what are the userspace visible impacts of this change. 
Which makes it hard for others to determine whether -stable kernels
should be patched.

> Fixes: 7da4e2cb8b1f ("mm: thp: kill __transhuge_page_enabled()")
> Reported-by: Saurabh Singh Sengar <ssengar@microsoft.com>

It's nice to include a Closes: link after a Reported-by:.  Then readers
are better able to answer the above question.

> Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: David Hildenbrand <david@redhat.com>