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@xxxxxxxxxxxxx> 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
