Re: [patch] mm, thp: always specify ineligible vmas as nh in smaps
From: Vlastimil Babka <hidden>
Date: 2018-09-24 18:27:58
Also in:
linux-mm, lkml
+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@google.com>
Fixes: 1860033237d4 ("mm: make PR_SET_THP_DISABLE immediately active")
Not worth for stable IMO, but makes sense otherwise.
Acked-by: Vlastimil Babka <redacted>
A question below:
quoted hunk
--- 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, ' ');