Re: [PATCH v12 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Michał Mirosław]

On Thu, 6 Apr 2023 at 23:12, Muhammad Usama Anjum
<usama.anjum@xxxxxxxxxxxxx> wrote:
> On 4/7/23 1:12 AM, Michał Mirosław wrote:
> > On Thu, 6 Apr 2023 at 09:40, Muhammad Usama Anjum
> > <usama.anjum@xxxxxxxxxxxxx> wrote:
> > [...]
> >> --- a/fs/proc/task_mmu.c
> >> +++ b/fs/proc/task_mmu.c
> > [...]
> >> +static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
> >> +                                 unsigned long end, struct mm_walk *walk)
> >> +{
[...]
> >> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >> +       ptl = pmd_trans_huge_lock(pmd, vma);
> >> +       if (ptl) {
> > [...]
> >> +               return ret;
> >> +       }
> >> +process_smaller_pages:
> >> +       if (pmd_trans_unstable(pmd))
> >> +               return 0;
> >
> > Why pmd_trans_unstable() is needed here and not only after split_huge_pmd()?
> I'm not entirely sure. But the idea is if THP is unstable, we should
> return. As it doesn't seem like after splitting THP can be unstable, we
> should not check it. Do you agree with the following?

The description of pmd_trans_unstable() [1] seems to indicate that it
is needed only after split_huge_pmd().

[1] https://elixir.bootlin.com/linux/v6.3-rc5/source/include/linux/pgtable.h#L1394

Best Regards
Michał Mirosław