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

[Peter Xu <peterx@redhat.com>]

On Wed, Aug 10, 2022 at 11:37:13AM +0200, David Hildenbrand wrote:
> On 09.08.22 00:08, Peter Xu wrote:
> > On Mon, Aug 08, 2022 at 04:21:39PM -0400, Peter Xu wrote:
> >> On Mon, Aug 08, 2022 at 06:25:21PM +0200, David Hildenbrand wrote:
> >>>>> 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.
> >>>
> >>> Thus the VM_WRITE check. :)
> >>>
> >>> I wonder if we should just do it cleanly and introduce the maybe_mkwrite
> >>> semantics here as well. Then there is no need for additional VM_WRITE
> >>> checks and hugetlb will work just like !hugetlb.
> >>
> >> Hmm yeah I think the VM_MAYSHARE check is correct, since we'll need to fail
> >> the cases where MAYSHARE && !SHARE - we used to silently let it pass.
> > 
> > Sorry I think this is a wrong statement I made..  IIUC we'll fail correctly
> > with/without the patch on any write to hugetlb RO regions.
> > 
> > Then I just don't see a difference on checking VM_SHARED or VM_MAYSHARE
> > here, it's just that VM_MAYSHARE check should work too like VM_SHARED so I
> > don't see a problem.
> > 
> > It also means I can't think of any valid case of having VM_WRITE when
> > reaching here, then the WARN_ON_ONCE() is okay but maybe also redundant.
> > Using maybe_mkwrite() seems misleading to me if FOLL_FORCE not ready for
> > hugetlbfs after all.
> > 
> 
> The main reason we'd have it would be to scream out lout and fail
> gracefully if someone would -- for example -- use it for something like
> FOLL_FORCE.

Having that WARN_ON_ONCE() is okay to me, but just to double check we're on
the same page: why there's concern on using FOLL_FORCE? IIUC we're talking
about shared mappings here, then no FOLL_FORCE possible at all?  IOW,
"!is_cow_mapping()" should fail in check_vma_flags() already.

The other thing is I'm wondering whether patch 2 should be postponed anyway
so that we can wait for a full resolution of the problem from Mike.

Thanks,

-- 
Peter Xu