linux-kernel - Re: PROBLEM: Remapping hugepages mappings causes kernel to return EINVAL

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Message-ID: <20171023165717.qx5qluryshz62zv5@dhcp22.suse.cz>
Date:   Mon, 23 Oct 2017 18:57:17 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "C.Wehrmeyer" <c.wehrmeyer@....de>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: PROBLEM: Remapping hugepages mappings causes kernel to return
 EINVAL

On Mon 23-10-17 18:46:59, C.Wehrmeyer wrote:
> On 23-10-17 18:13, Michal Hocko wrote:
> > On Mon 23-10-17 16:00:13, C.Wehrmeyer wrote:
> > > And just to be very sure I've added:
> > > 
> > > if (madvise(buf1,ALLOC_SIZE_1,MADV_HUGEPAGE)) {
> > >          errno_tmp = errno;
> > >          fprintf(stderr,"madvise: %u\n",errno_tmp);
> > >          goto out;
> > > }
> > > 
> > > /*Make sure the mapping is actually used*/
> > > memset(buf1,'!',ALLOC_SIZE_1);
> > 
> > Is the buffer aligned to 2MB?
> 
> When I omit MAP_HUGETLB for the flags that mmap receives - no.
> 
> #define ALLOC_SIZE_1 (2 * 1024 * 1024)
> [...]
> buf1 = mmap (
>         NULL,
>         ALLOC_SIZE_1,
>         prot, /*PROT_READ | PROT_WRITE*/
>         flags /*MAP_PRIVATE | MAP_ANONYMOUS*/,
>         -1,
>         0
> );
> 
> In such a case buf1 usually contains addresses which are aligned to 4 KiBs,
> such as 0x7f07d76e9000. 2-MiB-aligned addresses, such as 0x7f89f5e00000, are
> only produced with MAP_HUGETLB - which, if I understood the documentation
> correctly, is not the point of THPs as they are supposed to be transparent.

yes. You can use posix_memalign or you can mmap a larger block and
munmap the initial unaligned part.

[...]
-- 
Michal Hocko
SUSE Labs