On Thu, Dec 19, 2024 at 02:05:04PM +0100, David Hildenbrand wrote:
> On 23.11.24 00:23, Joanne Koong wrote:
> > For migrations called in MIGRATE_SYNC mode, skip migrating the folio if
> > it is under writeback and has the AS_WRITEBACK_INDETERMINATE flag set on its
> > mapping. If the AS_WRITEBACK_INDETERMINATE flag is set on the mapping, the
> > writeback may take an indeterminate amount of time to complete, and
> > waits may get stuck.
> >
> > Signed-off-by: Joanne Koong <joannelkoong@xxxxxxxxx>
> > Reviewed-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> > ---
> > mm/migrate.c | 5 ++++-
> > 1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index df91248755e4..fe73284e5246 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1260,7 +1260,10 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
> > */
> > switch (mode) {
> > case MIGRATE_SYNC:
> > - break;
> > + if (!src->mapping ||
> > + !mapping_writeback_indeterminate(src->mapping))
> > + break;
> > + fallthrough;
> > default:
> > rc = -EBUSY;
> > goto out;
>
> Ehm, doesn't this mean that any fuse user can essentially completely block
> CMA allocations, memory compaction, memory hotunplug, memory poisoning... ?!
>
> That sounds very bad.
The page under writeback are already unmovable while they are under
writeback. This patch is only making potentially unrelated tasks to
synchronously wait on writeback completion for such pages which in worst
case can be indefinite. This actually is solving an isolation issue on a
multi-tenant machine.
