Re: [PATCH 14/15] mm: Align THP mappings for non-DAX

Kirill A. Shutemov
2019-10-01T11:32:19Z

On Tue, Oct 01, 2019 at 05:21:26AM -0600, William Kucharski wrote:
>
>
> > On Oct 1, 2019, at 4:45 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> >
> > On Tue, Sep 24, 2019 at 05:52:13PM -0700, Matthew Wilcox wrote:
> >>
> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >> index cbe7d0619439..670a1780bd2f 100644
> >> --- a/mm/huge_memory.c
> >> +++ b/mm/huge_memory.c
> >> @@ -563,8 +563,6 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
> >>
> >> if (addr)
> >> goto out;
> >> - if (!IS_DAX(filp->f_mapping->host) || !IS_ENABLED(CONFIG_FS_DAX_PMD))
> >> - goto out;
> >>
> >> addr = __thp_get_unmapped_area(filp, len, off, flags, PMD_SIZE);
> >> if (addr)
> >
> > I think you reducing ASLR without any real indication that THP is relevant
> > for the VMA. We need to know if any huge page allocation will be
> > *attempted* for the VMA or the file.
>
> Without a properly aligned address the code will never even attempt allocating
> a THP.
>
> I don't think rounding an address to one that would be properly aligned to map
> to a THP if possible is all that detrimental to ASLR and without the ability to
> pick an aligned address it's rather unlikely anyone would ever map anything to
> a THP unless they explicitly designate an address with MAP_FIXED.
>
> If you do object to the slight reduction of the ASLR address space, what
> alternative would you prefer to see?

We need to know by the time if THP is allowed for this
file/VMA/process/whatever. Meaning that we do not give up ASLR entropy for
nothing.

For instance, if THP is disabled globally, there is no reason to align the
VMA to the THP requirements.

--
Kirill A. Shutemov