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Subject[patch] mm, thp: always specify ineligible vmas as nh in smaps
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>
---
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;
+
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, ' ');
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