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

[Yang Shi <shy828301@gmail.com>]

On Mon, Sep 25, 2023 at 1:01 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, 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.

Looks good to me. Reviewed-by: Yang Shi <shy828301@gmail.com>

>
> 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>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: David Hildenbrand <david@redhat.com>
> ---
> I've updated the changelog to reflect discussions in [1] -- leaving
> ack to David / Matthew on whether to take the patch.
>
> Changed from v3[1]:
>         - [akpm / David H. / M. Wilcox] Updated log to capture email discussion
> Changed from v2[2]:
>         - Fixed false negative in smaps check when !dax && ->huge_fault
> Changed from v1[3]:
>         - [Saurabhi] Allow ->huge_fault handler to handle fault, if it exists
>
> [1] https://lore.kernel.org/linux-mm/20230821234844.699818-1-zokeefe@google.com/
> [2] https://lore.kernel.org/linux-mm/20230818211533.2523697-1-zokeefe@google.com/
> [3] https://lore.kernel.org/linux-mm/CAAa6QmQw+F=o6htOn=6ADD6mwvMO=Ow_67f3ifBv3GpXx9Xg_g@mail.gmail.com/
>
> ---
>  mm/huge_memory.c | 20 +++++++++++++-------
>  1 file changed, 13 insertions(+), 7 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0f93a73115f73..797fe617e51ab 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -100,11 +100,11 @@ 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))
>                 return false;
>
>         /*
> @@ -132,12 +132,18 @@ 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))
> +       if (!vma_is_anonymous(vma)) {
> +               /*
> +                * Trust that ->huge_fault() handlers know what they are doing
> +                * in fault path.
> +                */
> +               if (((in_pf || smaps)) && vma->vm_ops->huge_fault)
> +                       return true;
> +               /* Only regular file is valid in collapse path */
> +               if (((!in_pf || smaps)) && file_thp_enabled(vma))
> +                       return true;
>                 return false;
> +       }
>
>         if (vma_is_temporary_stack(vma))
>                 return false;
> --
> 2.42.0.515.g380fc7ccd1-goog
>