Re: [PATCH v7 1/3] mm/mmap: separate writenotify and dirty tracking logic

[Lorenzo Stoakes]

On Tue, May 02, 2023 at 05:53:46PM +0100, Lorenzo Stoakes wrote:
> On Tue, May 02, 2023 at 06:38:53PM +0200, David Hildenbrand wrote:
> > On 02.05.23 18:34, Lorenzo Stoakes wrote:
> > > vma_wants_writenotify() is specifically intended for setting PTE page table
> > > flags, accounting for existing PTE flag state and whether that might
> > > already be read-only while mixing this check with a check whether the
> > > filesystem performs dirty tracking.
> > >
> > > Separate out the notions of dirty tracking and a PTE write notify checking
> > > in order that we can invoke the dirty tracking check from elsewhere.
> > >
> > > Note that this change introduces a very small duplicate check of the
> > > separated out vm_ops_needs_writenotify(). This is necessary to avoid making
> > > vma_needs_dirty_tracking() needlessly complicated (e.g. passing a
> > > check_writenotify flag or having it assume this check was already
> > > performed). This is such a small check that it doesn't seem too egregious
> > > to do this.
> > >
> > > Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> > > Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
> > > Reviewed-by: Mika Penttilä <mpenttil@xxxxxxxxxx>
> > > Reviewed-by: Jan Kara <jack@xxxxxxx>
> > > Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > ---
> > >   include/linux/mm.h |  1 +
> > >   mm/mmap.c          | 36 +++++++++++++++++++++++++++---------
> > >   2 files changed, 28 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index 27ce77080c79..7b1d4e7393ef 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -2422,6 +2422,7 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
> > >   #define  MM_CP_UFFD_WP_ALL                 (MM_CP_UFFD_WP | \
> > >   					    MM_CP_UFFD_WP_RESOLVE)
> > > +bool vma_needs_dirty_tracking(struct vm_area_struct *vma);
> > >   int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot);
> > >   static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma)
> > >   {
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index 5522130ae606..295c5f2e9bd9 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -1475,6 +1475,31 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
> > >   }
> > >   #endif /* __ARCH_WANT_SYS_OLD_MMAP */
> > > +/* Do VMA operations imply write notify is required? */
> > > +static bool vm_ops_needs_writenotify(const struct vm_operations_struct *vm_ops)
> > > +{
> > > +	return vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite);
> > > +}
> > > +
> > > +/*
> > > + * Does this VMA require the underlying folios to have their dirty state
> > > + * tracked?
> > > + */
> > > +bool vma_needs_dirty_tracking(struct vm_area_struct *vma)
> > > +{
> >
> > Sorry for not noticing this earlier, but ...
>
> pints_owed++
>
> >
> > what about MAP_PRIVATE mappings? When we write, we populate an anon page,
> > which will work as expected ... because we don't have to notify the fs?
> >
> > I think you really also want the "If it was private or non-writable, the
> > write bit is already clear */" part as well and remove "false" in that case.
> >
>
> Not sure a 'write bit is already clear' case is relevant to checking
> whether a filesystem dirty tracks? That seems specific entirely to the page
> table bits.
>
> That's why I didn't include it,
>
> A !VM_WRITE shouldn't be GUP-writable except for FOLL_FORCE, and that
> surely could be problematic if VM_MAYWRITE later?
>
> Thinking about it though a !VM_SHARE should probably can be safely assumed
> to not be dirty-trackable, so we probably do need to add a check for
> !VM_SHARED -> !vma_needs_dirty_tracking
>

On second thoughts, we explicitly check FOLL_FORCE && !is_cow_mapping() in
check_vma_flags() so that case cannot occur.

So actually yes we should probably include this on the basis of that and
the fact that a FOLL_WRITE operation will CoW the MAP_PRIVATE mapping.

This was an (over)abundance of caution.

Will fix on respin.

> > Or was there a good reason to disallow private mappings as well?
> >
>
> Until the page is CoW'd walking the page tables will get you to the page
> cache page right? This was the reason I (perhaps rather too quickly) felt
> MAP_PRIVATE should be excluded.
>
> However a FOLL_WRITE would trigger CoW... and then we'd be trivially OK.
>
> So yeah, ok perhaps I dismissed that a little too soon. I was concerned
> about some sort of egregious FOLL_FORCE case where somehow we'd end up with
> the page cache folio. But actually, that probably can't happen...
>
> > --
> > Thanks,
> >
> > David / dhildenb
> >