On 4/24/19 9:17 AM, Vlastimil Babka wrote:
On 4/24/19 5:47 PM, Yang Shi wrote:
On 4/24/19 6:10 AM, Vlastimil Babka wrote:
On 4/23/19 6:43 PM, Yang Shi wrote:
The commit 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each
vma") introduced THPeligible bit for processes' smaps. But, when checking
the eligibility for shmem vma, __transparent_hugepage_enabled() is
called to override the result from shmem_huge_enabled(). It may result
in the anonymous vma's THP flag override shmem's. For example, running a
simple test which create THP for shmem, but with anonymous THP disabled,
when reading the process's smaps, it may show:
7fc92ec00000-7fc92f000000 rw-s 00000000 00:14 27764 /dev/shm/test
Size: 4096 kB
...
[snip]
...
ShmemPmdMapped: 4096 kB
But how does this happen in the first place?
In __handle_mm_fault() we do:
if (pmd_none(*vmf.pmd) && __transparent_hugepage_enabled(vma)) {
ret = create_huge_pmd(&vmf);
if (!(ret & VM_FAULT_FALLBACK))
return ret;
And __transparent_hugepage_enabled() checks the global THP settings.
If THP is not enabled / is only for madvise and the vma is not madvised,
then this should fail, and also khugepaged shouldn't either run at all,
or don't do its job for such non-madvised vma.
If __transparent_hugepage_enabled() returns false, the code will not
reach create_huge_pmd() at all. If it returns true, create_huge_pmd()
actually will return VM_FAULT_FALLBACK for shmem since shmem doesn't
have huge_fault (or pmd_fault in earlier versions) method.
Then it will get into handle_pte_fault(), finally shmem_fault() is
called, which allocates THP by checking some global flag (i.e.
VM_NOHUGEPAGE and MMF_DISABLE_THP) and shmem THP knobs.
Aha, thanks! What a mess...
Yes, it does look convoluted. I'm wondering we may consider refactor the
shmem THP fault handling.
4.8 (the first version has shmem THP merged) behaves exactly in the same
way. So, I suspect this may be intended behavior.
Still looks like an oversight to me. And it's inconsistent... it might
fault huge shmem pages when THPs are globally disabled, but khugepaged
is still not running. I think it should just check the global THP flags
as well...
It does looks inconsistent, particularly for the khugepaged part.
