Re: [PATCH] mm: khugepaged: skip huge page collapse for special files

[Yang Shi <shy828301@gmail.com>]

On Wed, Oct 27, 2021 at 1:35 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Oct 27, 2021, at 12:52 PM, Yang Shi <shy828301@gmail.com> wrote:
> >
> > The read-only THP for filesystems would collapse THP for file opened
> > readonly and mapped with VM_EXEC, the intended usecase is to avoid TLB
> > miss for large text segment.  But it doesn't restrict the file types so
> > THP could be collapsed for non-regular file, for example, block device,
> > if it is opened readonly and mapped with EXEC permission.  This may
> > cause bugs, like [1] and [2].
> >
> > This is definitely not intended usecase, so just collapsing THP for regular
> > file in order to close the attack surface.
> >
> > [1] https://lore.kernel.org/lkml/CACkBjsYwLYLRmX8GpsDpMthagWOjWWrNxqY6ZLNQVr6yx+f5vA@mail.gmail.com/
> > [2] https://lore.kernel.org/linux-mm/000000000000c6a82505ce284e4c@google.com/
> >
> > Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> > Reported-by: Hao Sun <sunhao.th@gmail.com>
> > Reported-by: syzbot+aae069be1de40fb11825@syzkaller.appspotmail.com
> > Cc: Hao Sun <sunhao.th@gmail.com>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > Cc: Song Liu <songliubraving@fb.com>
> > Cc: Andrea Righi <andrea.righi@canonical.com>
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > ---
> > The patch is basically based off the proposal from Hugh
> > (https://lore.kernel.org/linux-mm/a07564a3-b2fc-9ffe-3ace-3f276075ea5c@google.com/).
> > It seems Hugh is too busy to prepare the patch for formal submission (I
> > didn't hear from him by pinging him a couple of times on mailing list),
> > so I prepared the patch and added his SOB.
> >
> > mm/khugepaged.c | 17 ++++++++++-------
> > 1 file changed, 10 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 045cc579f724..e91b7271275e 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -445,22 +445,25 @@ static bool hugepage_vma_check(struct vm_area_struct *vma,
> >       if (!transhuge_vma_enabled(vma, vm_flags))
> >               return false;
> >
> > -     /* Enabled via shmem mount options or sysfs settings. */
> > -     if (shmem_file(vma->vm_file) && shmem_huge_enabled(vma)) {
> > +     if (vma->vm_file)
> >               return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
> >                               HPAGE_PMD_NR);
>
> Am I misreading this? If we return here for vma->vm_file, the following
> logic (shmem_file(), etc.) would be skipped, no?

Oh, yes, you are right. My mistake.

Andrew,

Could you please apply the below fix?

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index e91b7271275e..26f1798c88d2 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -445,9 +445,9 @@ static bool hugepage_vma_check(struct vm_area_struct *vma,
        if (!transhuge_vma_enabled(vma, vm_flags))
                return false;

-       if (vma->vm_file)
-               return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
-                               HPAGE_PMD_NR);
+       if (vma->vm_file && !IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) -
+                               vma->vm_pgoff, HPAGE_PMD_NR))
+               return false;

        /* Enabled via shmem mount options or sysfs settings. */
        if (shmem_file(vma->vm_file))

>
> Thanks,
> Song
>
> > -     }
> > +
> > +     /* Enabled via shmem mount options or sysfs settings. */
> > +     if (shmem_file(vma->vm_file))
> > +             return shmem_huge_enabled(vma);
> >
> >       /* THP settings require madvise. */
> >       if (!(vm_flags & VM_HUGEPAGE) && !khugepaged_always())
> >               return false;
> >
> > -     /* Read-only file mappings need to be aligned for THP to work. */
> > +     /* Only regular file is valid */
> >       if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && vma->vm_file &&
> > -         !inode_is_open_for_write(vma->vm_file->f_inode) &&
> >           (vm_flags & VM_EXEC)) {
> > -             return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
> > -                             HPAGE_PMD_NR);
> > +             struct inode *inode = vma->vm_file->f_inode;
> > +
> > +             return !inode_is_open_for_write(inode) &&
> > +                     S_ISREG(inode->i_mode);
> >       }
> >
> >       if (!vma->anon_vma || vma->vm_ops)
> > --
> > 2.26.2
> >
>