On Mon, 25 Sep 2023 13:01:10 -0700 "Zach O'Keefe" <zokeefe@xxxxxxxxxx> 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@xxxxxxxxxxxxx>
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@xxxxxxxxxx>
> Cc: Yang Shi <shy828301@xxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
