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

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

> -----Original Message-----
> From: Zach O'Keefe <zokeefe@google.com>
> Sent: Tuesday, August 22, 2023 5:19 AM
> To: linux-mm@kvack.org; Yang Shi <shy828301@gmail.com>
> Cc: linux-kernel@vger.kernel.org; Zach O'Keefe <zokeefe@google.com>;
> Saurabh Singh Sengar <ssengar@microsoft.com>
> Subject: [EXTERNAL] [PATCH v3] mm/thp: fix "mm: thp: kill
> __transhuge_page_enabled()"
>
> [You don't often get email from zokeefe@google.com. Learn why this is
> important at https://aka.ms/LearnAboutSenderIdentification ]
>
> 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 v2[1]:
>         - Fixed false negative in smaps check when !dax && ->huge_fault
> Changed from v1[2]:
>         - [Saurabhi] Allow ->huge_fault handler to handle fault, if it exists
>
> There are some logical holes in smaps' THPeligible checks here, but those are
> best dealt with in follow-up patches.  For now, just make sure the fault path is
> dealt with.
>
> [1]
> https://lore.k/
> ernel.org%2Flinux-mm%2F20230818211533.2523697-1-
> zokeefe%40google.com%2F&data=05%7C01%7Cssengar%40microsoft.com%7
> Ce782558e7bce4f9d060608dba2a12b58%7C72f988bf86f141af91ab2d7cd011
> db47%7C1%7C0%7C638282585367952964%7CUnknown%7CTWFpbGZsb3d8e
> yJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D
> %7C3000%7C%7C%7C&sdata=t%2FwAGlOyKmKp%2FnDPGv9cl2j3h%2F3xVuV
> Y%2BQqeu3A4HHk%3D&reserved=0
> [2]
> https://lore.k/
> ernel.org%2Flinux-
> mm%2FCAAa6QmQw%2BF%3Do6htOn%3D6ADD6mwvMO%3DOw_67f3ifBv3
> GpXx9Xg_g%40mail.gmail.com%2F&data=05%7C01%7Cssengar%40microsoft.
> com%7Ce782558e7bce4f9d060608dba2a12b58%7C72f988bf86f141af91ab2d
> 7cd011db47%7C1%7C0%7C638282585367952964%7CUnknown%7CTWFpbGZ
> sb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6M
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> QMu58gnKG7aLo%3D&reserved=0
>
> ---
>  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
> eb3678360b97..901dcf8db8d2 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))
>                 return false;
>
>         /*
> @@ -128,12 +128,18 @@ 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;

Thanks for the patch. I have tested it, looks good to me.

- Saurabh

> +               /* 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.rc1.204.g551eb34607-goog