Re: [PATCH v2 1/2] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

[Yang Shi <shy828301@gmail.com>]

On Fri, Aug 18, 2023 at 2:15 PM 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, 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.
>
> Fixes: 7da4e2cb8b1f ("mm: thp: kill __transhuge_page_enabled()")
> Reported-by: Saurabh Singh Sengar <ssengar@microsoft.com>
> Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> Cc: Yang Shi <shy828301@gmail.com>
> ---
> Changed from v1[1]:
>         - [Saurabhi] Allow ->huge_fault handler to handle fault, if it exists
>
> [1] https://lore.kernel.org/linux-mm/CAAa6QmQw+F=o6htOn=6ADD6mwvMO=Ow_67f3ifBv3GpXx9Xg_g@mail.gmail.com/

Thanks, Zach. The patch looks correct to me. You can add
Reviewed-by:Yang Shi <shy828301@gmail.com>.

A further comment below...

>
> ---
>  mm/huge_memory.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index eb3678360b97..cd379b2c077b 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -96,11 +96,11 @@ bool hugepage_vma_check(struct vm_area_struct *vma, unsigned long vm_flags,
>                 return in_pf;
>
>         /*
> -        * Special VMA and hugetlb VMA.
> +        * khugepaged special VMA and hugetlb VMA.
>          * Must be checked after dax since some dax mappings may have
>          * VM_MIXEDMAP set.
>          */
> -       if (vm_flags & VM_NO_KHUGEPAGED)
> +       if (!in_pf && !smaps && (vm_flags & VM_NO_KHUGEPAGED))

I'm wondering whether we shall remove VM_MIXEDMAP from
VM_NO_KHUGEPAGED or not if that kind VMAs are huge page applicable for
some usecases. The downside may be some CPU time waste on the
VM_MIXEDMAP area which has PFN instead of struct page, but it should
be ok. Anything else did I miss? Just back from a long vacation, my
brain is still not running in full speed yet.

>                 return false;
>
>         /*
> @@ -128,12 +128,15 @@ bool hugepage_vma_check(struct vm_area_struct *vma, unsigned long vm_flags,
>                                            !hugepage_flags_always())))
>                 return false;
>
> -       /* Only regular file is valid */
> -       if (!in_pf && file_thp_enabled(vma))
> -               return true;
> -
>         if (!vma_is_anonymous(vma))
> -               return false;
> +               return in_pf ?
> +                       /*
> +                        * Trust that ->huge_fault() handlers know
> +                        * what they are doing in fault path.
> +                        */
> +                       !!vma->vm_ops->huge_fault :
> +                       /* Only regular file is valid in collapse path */
> +                       file_thp_enabled(vma);
>
>         if (vma_is_temporary_stack(vma))
>                 return false;
> --
> 2.42.0.rc1.204.g551eb34607-goog
>