Re: [PATCH v16 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Peter Xu <peterx@redhat.com>]

On Fri, Jun 02, 2023 at 04:18:38PM +0500, Muhammad Usama Anjum wrote:
> On 6/2/23 1:11 AM, Peter Xu wrote:
> > On Thu, Jun 01, 2023 at 01:16:14PM +0500, Muhammad Usama Anjum wrote:
> >> On 6/1/23 2:46 AM, Peter Xu wrote:
> >>> Muhammad,
> >>>
> >>> Sorry, I probably can only review the non-interface part, and leave the
> >>> interface/buffer handling, etc. review for others and real potential users
> >>> of it..
> >> Thank you so much for the review. I think mostly we should be okay with
> >> interface as everybody has been making suggestions over the past revisions.
> >>
> >>>
> >>> On Thu, May 25, 2023 at 01:55:14PM +0500, Muhammad Usama Anjum wrote:
> >>>> +static inline void make_uffd_wp_huge_pte(struct vm_area_struct *vma,
> >>>> +					 unsigned long addr, pte_t *ptep,
> >>>> +					 pte_t ptent)
> >>>> +{
> >>>> +	pte_t old_pte;
> >>>> +
> >>>> +	if (!huge_pte_none(ptent)) {
> >>>> +		old_pte = huge_ptep_modify_prot_start(vma, addr, ptep);
> >>>> +		ptent = huge_pte_mkuffd_wp(old_pte);
> >>>> +		ptep_modify_prot_commit(vma, addr, ptep, old_pte, ptent);
> >>>
> >>> huge_ptep_modify_prot_start()?
> >> Sorry, I didn't realized that huge_ptep_modify_prot_start() is different
> >> from its pte version.
> > 
> > Here I meant huge_ptep_modify_prot_commit()..
> I'll update.
> 
> > 
> >>
> >>>
> >>> The other thing is what if it's a pte marker already?  What if a hugetlb
> >>> migration entry?  Please check hugetlb_change_protection().
> >> I've updated it in more better way. Please let me know what do you think
> >> about the following:
> >>
> >> static inline void make_uffd_wp_huge_pte(struct vm_area_struct *vma,
> >> 					 unsigned long addr, pte_t *ptep,
> >> 					 pte_t ptent)
> >> {
> >> 	if (is_hugetlb_entry_hwpoisoned(ptent) || is_pte_marker(ptent))
> >> 		return;
> >>
> >> 	if (is_hugetlb_entry_migration(ptent))
> >> 		set_huge_pte_at(vma->vm_mm, addr, ptep,
> >> 				pte_swp_mkuffd_wp(ptent));
> >> 	else if (!huge_pte_none(ptent))
> >> 		ptep_modify_prot_commit(vma, addr, ptep, ptent,
> >> 					huge_pte_mkuffd_wp(ptent));
> >> 	else
> >> 		set_huge_pte_at(vma->vm_mm, addr, ptep,
> >> 				make_pte_marker(PTE_MARKER_UFFD_WP));
> >> }
> > 
> > the is_pte_marker() check can be extended to double check
> > pte_marker_uffd_wp() bit, but shouldn't matter a lot since besides the
> > uffd-wp bit currently we only support swapin error which should sigbus when
> > accessed, so no point in tracking anyway.
> Yeah, we are good with what we have as even if more bits are supported in
> pte markers, this function is only reached when UNPOPULATED + ASYNC WP are
> enabled. So no other bit would be set on the marker.

I think we don't know?  swapin error bit can be set there afaiu, if someone
swapoff -a and found a swap device broken for some swapped out ptes.

But again I think that's fine for now.

> 
> > 
> >>
> >> As we always set UNPOPULATED, so markers are always set on none ptes
> >> initially. Is it possible that a none pte becomes present, then swapped and
> >> finally none again? So I'll do the following addition for make_uffd_wp_pte():
> >>
> >> --- a/fs/proc/task_mmu.c
> >> +++ b/fs/proc/task_mmu.c
> >> @@ -1800,6 +1800,9 @@ static inline void make_uffd_wp_pte(struct
> >> vm_area_struct *vma,
> >>  	} else if (is_swap_pte(ptent)) {
> >>  		ptent = pte_swp_mkuffd_wp(ptent);
> >>  		set_pte_at(vma->vm_mm, addr, pte, ptent);
> >> +	} else {
> >> +		set_pte_at(vma->vm_mm, addr, pte,
> >> +			   make_pte_marker(PTE_MARKER_UFFD_WP));
> >>  	}
> >>  }
> > 
> > Makes sense, you can leverage userfaultfd_wp_use_markers() here, and you
> > should probably keep the protocol (only set the marker when WP_UNPOPULATED
> > for anon).
> This function is only reachable when UNPOPULATED + Async WP are set. So we
> don't need to use userfaultfd_wp_use_markers().

I don't remember where you explicitly checked that to make sure it'll
always be the case, but if you do it'll still be nice if you can add a
comment right above here explaining.

Or, maybe you can still use userfaultfd_wp_use_markers() here, then you can
just WARN_ON_ONCE() on it, which looks even better?

[...]

> > Hmm, is it a bug for pagemap?  pagemapread.buffer should be linear to the
> > address range to be scanned to me.  If it skips some unstable pmd without
> > filling in anything it seems everything later will be shifted with
> > PMD_SIZE..  I had a feeling that it should set walk->action==ACTION_AGAIN
> > before return.
> I don't think this is a bug if this is how it was implemented in the first
> place. In this task_mmu.c file, we can find several examples of the same
> pattern that error isn't returned if pmd_trans_unstable() succeeds.

I don't see why multiple same usages mean it's correct.. maybe they're all
buggy?

I can post a patch for this to collect opinions to see if I missed
something. I'd suggest you figure out what's the right thing to do for the
new interface and make it right from the start, no matter how it was
implemented elsewhere.

Thanks,

-- 
Peter Xu