Re: [PATCH v1 2/2] mm/hugetlb: support write-faults in shared mappings

[Peter Xu]

On Fri, Aug 05, 2022 at 08:20:52PM +0200, David Hildenbrand wrote:
> On 05.08.22 20:12, Peter Xu wrote:
> > On Fri, Aug 05, 2022 at 01:03:29PM +0200, David Hildenbrand wrote:
> >> Let's add a safety net if we ever get (again) a write-fault on a R/O-mapped
> >> page in a shared mapping, in which case we simply have to map the
> >> page writable.
> >>
> >> VM_MAYSHARE handling in hugetlb_fault() for FAULT_FLAG_WRITE
> >> indicates that this was at least envisioned, but could never have worked
> >> as expected. This theoretically paves the way for softdirty tracking
> >> support in hugetlb.
> >>
> >> Tested without the fix for softdirty tracking.
> >>
> >> Note that there is no need to do any kind of reservation in hugetlb_fault()
> >> in this case ... because we already have a hugetlb page mapped R/O
> >> that we will simply map writable and we are not dealing with COW/unsharing.
> >>
> >> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> >> ---
> >>  mm/hugetlb.c | 21 ++++++++++++++-------
> >>  1 file changed, 14 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> >> index a18c071c294e..bbab7aa9d8f8 100644
> >> --- a/mm/hugetlb.c
> >> +++ b/mm/hugetlb.c
> >> @@ -5233,6 +5233,16 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
> >>  	VM_BUG_ON(unshare && (flags & FOLL_WRITE));
> >>  	VM_BUG_ON(!unshare && !(flags & FOLL_WRITE));
> >>  
> >> +	/* Let's take out shared mappings first, this should be a rare event. */
> >> +	if (unlikely(vma->vm_flags & VM_MAYSHARE)) {
> > 
> > Should we check VM_SHARED instead?
> 
> Relying on VM_SHARED to detect MAP_PRIVATE vs. MAP_SHARED is
> unfortunately wrong.
> 
> If you're curious, take a look at f83a275dbc5c ("mm: account for
> MAP_SHARED mappings using VM_MAYSHARE and not VM_SHARED in hugetlbfs")
> and mmap() code.
> 
> Long story short: if the file is read-only, we only have VM_MAYSHARE but
> not VM_SHARED (and consequently also not VM_MAYWRITE).

To ask in another way: if file is RO but mapped RW (mmap() will have
VM_SHARED cleared but VM_MAYSHARE set), then if we check VM_MAYSHARE here
won't we grant write bit errornously while we shouldn't? As the user
doesn't really have write permission to the file.

> 
> > 
> >> +		if (unshare)
> >> +			return 0;
> > 
> > Curious when will this happen especially if we switch to VM_SHARED above.
> > Shouldn't "unshare" not happen at all on a shared region?
> 
> FAULT_FLAG_UNSHARE is documented to behave like:
> 
> "FAULT_FLAG_UNSHARE is ignored and treated like an ordinary read fault
> when no existing R/O-mapped anonymous page is encountered."
> 
> It should currently not happen. Focus on should ;)

OK. :)

Then does it also mean that it should be better to turn into
WARN_ON_ONCE()?  It's just that it looks like a valid path if without it.

> 
> > 
> >> +		if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE)))
> >> +			return VM_FAULT_SIGSEGV;
> > 
> > I had a feeling that you just want to double check we have write
> > permission, but IIUC this should be checked far earlier or we'll have
> > problem.  No strong opinion if so, but I'd suggest dropping this one,
> > otherwise we could add tons of WARN_ON_ONCE() in anywhere in the page fault
> > stack and they mostly won't trigger at all.
> 
> Not quite. We usually (!hugetlb) have maybe_mkwrite() all over the
> place. This is just an indication that we don't have maybe semantics
> here. But as we also don't have it for hugetlb anon code below, maybe I
> can just drop it. (or check it for both call paths)

Hmm, this reminded me to wonder how hugetlb handles FOLL_FORCE|FOLL_WRITE
on RO regions.

Maybe that check is needed, but however instead of warning and sigbus, we
need to handle it?

I'll need to read more on this later, but raise this up.

> 
> > 
> >> +		set_huge_ptep_writable(vma, haddr, ptep);
> > 
> > Do we wanna set dirty bits too?
> 
> set_huge_ptep_writable() handles that.

Right, I noticed it right after I sent too..

Thanks,

-- 
Peter Xu