Re: [PATCH 04/16] huge tmpfs: revert shmem's use of transhuge_vma_enabled()

On Sat, Jul 31, 2021 at 9:01 PM Hugh Dickins [off-list ref] wrote:

On Fri, 30 Jul 2021, Yang Shi wrote:

quoted

On Fri, Jul 30, 2021 at 12:36 AM Hugh Dickins [off-list ref] wrote:

quoted

5.14 commit e6be37b2e7bd ("mm/huge_memory.c: add missing read-only THP
checking in transparent_hugepage_enabled()") added transhuge_vma_enabled()
as a wrapper for two very different checks: shmem_huge_enabled() prefers
to show those two checks explicitly, as before.

Basically I have no objection to separating them again. But IMHO they
seem not very different. Or just makes things easier for the following
patches?

Well, it made it easier to apply the patch I'd prepared earlier,
but that was not the point; and I thought it best to be upfront
about the reversion, rather than hiding it in the movement.

The end result of the two checks is the same (don't try for huge pages),
and they have been grouped together because they occurred together in
several places, and both rely on "vma".

But one check is whether the app has marked that address range not to use
THPs; and the other check is whether the process is running in a hierarchy
that has been marked never to use THPs (which just uses vma to get to mm
to get to mm->flags (whether current->mm would be more relevant is not an
argument I want to get into, I'm not at all sure)).

To me those are very different; and I'm particularly concerned to make
MMF_DISABLE_THP references visible, since it did not exist when Kirill
and I first implemented shmem huge pages, and I've tended to forget it:
but consider it more in this series.

Yes, I agree one checks vma the other one checks mm, they are
different from this perspective. Anyway, as I said I have no objection
to this change. You could add Reviewed-by: Yang Shi
[off-list ref]

Hugh

quoted

quoted

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/shmem.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index ce3ccaac54d6..c6fa6f4f2db8 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c

@@ -4003,7 +4003,8 @@ bool shmem_huge_enabled(struct vm_area_struct *vma)
        loff_t i_size;
        pgoff_t off;

-       if (!transhuge_vma_enabled(vma, vma->vm_flags))
+       if ((vma->vm_flags & VM_NOHUGEPAGE) ||
+           test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
                return false;
        if (shmem_huge == SHMEM_HUGE_FORCE)
                return true;
--

2.26.2