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
> 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
>
