Re: [PATCH] /proc/PID/smaps: Add PMD migration entry parsing

[Konstantin Khlebnikov]

On 01/04/2020 05.31, Huang, Ying wrote:
> Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> writes:
>
> > On 31/03/2020 11.56, Huang, Ying wrote:
> > > From: Huang Ying <ying.huang@xxxxxxxxx>
> > >
> > > Now, when read /proc/PID/smaps, the PMD migration entry in page table is simply
> > > ignored.  To improve the accuracy of /proc/PID/smaps, its parsing and processing
> > > is added.
> > >
> > > Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> > > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > Cc: Zi Yan <ziy@xxxxxxxxxx>
> > > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > > Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> > > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > > Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> > > Cc: "JÃrÃme Glisse" <jglisse@xxxxxxxxxx>
> > > Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > > ---
> > >    fs/proc/task_mmu.c | 16 ++++++++++++----
> > >    1 file changed, 12 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > > index 8d382d4ec067..b5b3aef8cb3b 100644
> > > --- a/fs/proc/task_mmu.c
> > > +++ b/fs/proc/task_mmu.c
> > > @@ -548,8 +548,17 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
> > >    	bool locked = !!(vma->vm_flags & VM_LOCKED);
> > >    	struct page *page;
> >
> >          struct page *page = NULL;
>
> Looks good.  Will do this in the next version.
>
> > >    -	/* FOLL_DUMP will return -EFAULT on huge zero page */
> > > -	page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
> > > +	if (pmd_present(*pmd)) {
> > > +		/* FOLL_DUMP will return -EFAULT on huge zero page */
> > > +		page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
> > > +	} else if (unlikely(is_swap_pmd(*pmd))) {
> > > +		swp_entry_t entry = pmd_to_swp_entry(*pmd);
> > > +
> > > +		VM_BUG_ON(!is_migration_entry(entry));
> > > +		page = migration_entry_to_page(entry);
> >
> >                  if (is_migration_entry(entry))
> >                          page = migration_entry_to_page(entry);
> >
> > Seems safer and doesn't add much code.
>
> With this, we lose an opportunity to capture some bugs during debugging.
> Right?

You can keep VM_BUG_ON or VM_WARN_ON_ONCE

Off-by-page in statistics isn't a big deal and not a good reason to crash (even debug) kernel.
But for normal build should use safe behaviour if this isn't hard.

> Best Regards,
> Huang, Ying
>
> > > +	} else {
> > > +		return;
> > > +	}
> > >    	if (IS_ERR_OR_NULL(page))
> > >    		return;
> > >    	if (PageAnon(page))
> > > @@ -578,8 +587,7 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> > >      	ptl = pmd_trans_huge_lock(pmd, vma);
> > >    	if (ptl) {
> > > -		if (pmd_present(*pmd))
> > > -			smaps_pmd_entry(pmd, addr, walk);
> > > +		smaps_pmd_entry(pmd, addr, walk);
> > >    		spin_unlock(ptl);
> > >    		goto out;
> > >    	}