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version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Nov 26, 2025 at 05:44:58PM +0000, Matthew Wilcox (Oracle) wrote: > If we get a signal, we need to restore the vm_refcnt. We don't think > that the refcount can actually be decremented to zero here as it > requires the VMA to be detached, and the vma_mark_detached() uses > TASK_UNINTERRUPTIBLE. However, that's a bit subtle, so handle it > as if the refcount was zero at the start of this function. > > Reported-by: syzbot+5b19bad23ac7f44bf8b8@syzkaller.appspotmail.com > Fixes: 2197bb60f890 ("mm: add vma_start_write_killable()") > Signed-off-by: Matthew Wilcox (Oracle) > Cc: Suren Baghdasaryan > Cc: Liam R. Howlett > Cc: Vlastimil Babka > Cc: Lorenzo Stoakes > --- > mm/mmap_lock.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c > index e6e5570d1ec7..3c9bf2f96280 100644 > --- a/mm/mmap_lock.c > +++ b/mm/mmap_lock.c > @@ -74,6 +74,14 @@ static inline int __vma_enter_locked(struct vm_area_struct *vma, > refcount_read(&vma->vm_refcnt) == tgt_refcnt, > state); > if (err) { > + if (refcount_sub_and_test(VMA_LOCK_OFFSET, &vma->vm_refcnt)) { Really think we should WARN_ON_ONCE() as Vlasta suggested. It's an 'impossible' situation so we should make that clear. And we should find out about it if the impossible happens... :) > + /* > + * We got a fatal signal, but the last reader went > + * away as well. Resolve the race in favour of This is very subtle, I don't think this really explains this clearly enough. Maybe put something like: /* Couldn't wait on readers probably due to a fatal signal, so unlock. */ Before the refcount_sub_and_test() And: /* Shouldn't be possible - VMA entirely detached, so treat it as such. */ Before err = 0? > + * the vma being detached. > + */ > + err = 0; < pre-existing issue > As discussed off-list, this name is horrible. It's a value that returns 1 if attached and now locked, 0 if detached, or an error code if the RCU waiter fails. It's not an error code and this actually tripped me up reading this code. We can address it elsewhere however. > + } > rwsem_release(&vma->vmlock_dep_map, _RET_IP_); > return err; > } > -- > 2.47.2 > > Thanks, Lorenzo