+CC linux-mm linux-api
On 9/24/18 7:55 PM, David Rientjes wrote:
> Commit 1860033237d4 ("mm: make PR_SET_THP_DISABLE immediately active")
> introduced a regression in that userspace cannot always determine the set
> of vmas where thp is ineligible.
>
> Userspace relies on the "nh" flag being emitted as part of /proc/pid/smaps
> to determine if a vma is eligible to be backed by hugepages.
>
> Previous to this commit, prctl(PR_SET_THP_DISABLE, 1) would cause thp to
> be disabled and emit "nh" as a flag for the corresponding vmas as part of
> /proc/pid/smaps. After the commit, thp is disabled by means of an mm
> flag and "nh" is not emitted.
>
> This causes smaps parsing libraries to assume a vma is eligible for thp
> and ends up puzzling the user on why its memory is not backed by thp.
>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
Fixes: 1860033237d4 ("mm: make PR_SET_THP_DISABLE immediately active")
Not worth for stable IMO, but makes sense otherwise.
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
A question below:
> ---
> fs/proc/task_mmu.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -653,13 +653,23 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
> #endif
> #endif /* CONFIG_ARCH_HAS_PKEYS */
> };
> + unsigned long flags = vma->vm_flags;
> size_t i;
>
> + /*
> + * Disabling thp is possible through both MADV_NOHUGEPAGE and
> + * PR_SET_THP_DISABLE. Both historically used VM_NOHUGEPAGE. Since
> + * the introduction of MMF_DISABLE_THP, however, userspace needs the
> + * ability to detect vmas where thp is not eligible in the same manner.
> + */
> + if (vma->vm_mm && test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
> + flags |= VM_NOHUGEPAGE;
Should it also clear VM_HUGEPAGE? In case MMF_DISABLE_THP overrides a
madvise(MADV_HUGEPAGE)'d vma? (I expect it does?)
> +
> seq_puts(m, "VmFlags: ");
> for (i = 0; i < BITS_PER_LONG; i++) {
> if (!mnemonics[i][0])
> continue;
> - if (vma->vm_flags & (1UL << i)) {
> + if (flags & (1UL << i)) {
> seq_putc(m, mnemonics[i][0]);
> seq_putc(m, mnemonics[i][1]);
> seq_putc(m, ' ');
>
