On Fri, Apr 04, 2025 at 10:13:55PM +0200, David Hildenbrand wrote:
> On 04.04.25 22:09, Joanne Koong wrote:
> > On Fri, Apr 4, 2025 at 12:13 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > >
> > > On 04.04.25 20:14, Joanne Koong wrote:
> > > > Add a new mapping flag AS_WRITEBACK_INDETERMINATE which filesystems may
> > > > set to indicate that writing back to disk may take an indeterminate
> > > > amount of time to complete. Extra caution should be taken when waiting
> > > > on writeback for folios belonging to mappings where this flag is set.
> > > >
> > > > Signed-off-by: Joanne Koong <joannelkoong@xxxxxxxxx>
> > > > Reviewed-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> > > > Acked-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> > > > ---
> > > > include/linux/pagemap.h | 11 +++++++++++
> > > > 1 file changed, 11 insertions(+)
> > > >
> > > > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> > > > index 26baa78f1ca7..762575f1d195 100644
> > > > --- a/include/linux/pagemap.h
> > > > +++ b/include/linux/pagemap.h
> > > > @@ -210,6 +210,7 @@ enum mapping_flags {
> > > > AS_STABLE_WRITES = 7, /* must wait for writeback before modifying
> > > > folio contents */
> > > > AS_INACCESSIBLE = 8, /* Do not attempt direct R/W access to the mapping */
> > > > + AS_WRITEBACK_INDETERMINATE = 9, /* Use caution when waiting on writeback */
> > > > /* Bits 16-25 are used for FOLIO_ORDER */
> > > > AS_FOLIO_ORDER_BITS = 5,
> > > > AS_FOLIO_ORDER_MIN = 16,
> > > > @@ -335,6 +336,16 @@ static inline bool mapping_inaccessible(struct address_space *mapping)
> > > > return test_bit(AS_INACCESSIBLE, &mapping->flags);
> > > > }
> > > >
> > > > +static inline void mapping_set_writeback_indeterminate(struct address_space *mapping)
> > > > +{
> > > > + set_bit(AS_WRITEBACK_INDETERMINATE, &mapping->flags);
> > > > +}
> > > > +
> > > > +static inline bool mapping_writeback_indeterminate(struct address_space *mapping)
> > > > +{
> > > > + return test_bit(AS_WRITEBACK_INDETERMINATE, &mapping->flags);
> > > > +}
> > > > +
> > > > static inline gfp_t mapping_gfp_mask(struct address_space * mapping)
> > > > {
> > > > return mapping->gfp_mask;
> > >
> > > Staring at this again reminds me of my comment in [1]
> > >
> > > "
> > > b) Call it sth. like AS_WRITEBACK_MIGHT_DEADLOCK_ON_RECLAIM to express
> > > that very deadlock problem.
> > > "
> > >
> > > In the context here now, where we really only focus on the reclaim
> > > deadlock that can happen for trusted FUSE servers during reclaim, would
> > > it make sense to call it now something like that?
> >
> > Happy to make this change. My thinking was that
> > 'AS_WRITEBACK_INDETERMINATE' could be reused in the future for stuff
> > besides reclaim, but we can cross that bridge if that ends up being
> > the case.
>
> Yes, but I'm afraid one we start using it in other context we're reaching
> the point where we are trying to deal with untrusted user space and the page
> lock would already be a similar problem.
>
> Happy to be wrong on this one.
>
> Wait for other opinions first. Apart from that, no objection from my side.
I am on-board with keeping it specific to reclaim deadlock avoidance and
naming it such.
