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tests=bogofilter, spamicity=0.000000, 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 03:36:46PM +0100, Vlastimil Babka wrote: > On 11/26/25 5:28 AM, Suren Baghdasaryan wrote: > > On Tue, Nov 25, 2025 at 7:44 PM Matthew Wilcox (Oracle) > > wrote: > >> > >> If we get a signal, we need to restore the vm_refcnt. The wrinkle in > >> that is that we might be the last reference. If that happens, fix the > >> refcount to look like we weren't interrupted by a fatal signal. > >> > >> 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 > >> --- > >> Andrew, since the vma_start_write_killable() patch is in mm-stable, > >> I don't think you can put this in as a fixup, right? > >> > >> Suren, Liam, Vlastimil, Lorenzo ... none of you spotted this bug. > > > > Doh! This is embarassing... > > Hand-rolled synchronization primitives are wonderful, aren't they? I do wonder why we reworked the VMA locks again, I understood the previous iteration :) Not sure we fully justified that. But it's water under the bridge now. > > >> Any other stupid thing I've done? And am I doing the right thing > >> with refcount_set()? > > I think it's not wrong, but because it's dead code anyway. So it's > unnecessary. > > >> mm/mmap_lock.c | 9 +++++++++ > >> 1 file changed, 9 insertions(+) > >> > >> diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c > >> index e6e5570d1ec7..71af7f0a5fe1 100644 > >> --- a/mm/mmap_lock.c > >> +++ b/mm/mmap_lock.c > >> @@ -74,9 +74,18 @@ static inline int __vma_enter_locked(struct vm_area_struct *vma, > >> refcount_read(&vma->vm_refcnt) == tgt_refcnt, > >> state); > >> if (err) { OK so the sequence of events as I undersatnd it are: - VMA_LOCK_OFFSET added to vma->vm_refcnt (assuming not zero) - we try to wait on vma->vm_mm->vma_writer_wait - this fails because ___rcuwait_wait_event() calls signal_pending_state() which (if task is in interruptible mode) indicates a fatal signal, so -EINTR is returned. - Oopsies we need to subtract VMA_LOCK_OFFSET before we're done. - If the resulting reference count is zero, i.e. we held the last reference, we then... take the lock anyway? > >> + if (refcount_sub_and_test(VMA_LOCK_OFFSET, &vma->vm_refcnt)) { > >> + /* Oh cobblers. While we got a fatal signal, we Well for one this comment is broken ;) Plus 'cobblers'... when 'Gosh darn it' was there for the taking... > >> + * raced with the last user. Pretend we didn't notice > >> + * the signal > >> + */ I don't love that we just assume that rcu_wait_event() can and forever more only error out on fatal signal. I mean I guess probably it can but... maybe worth rewording to say we failed to wait so have to subtract but raced and probably if -EINTR this means... > >> + refcount_set(&vma->vm_refcnt, VMA_LOCK_OFFSET); > >> + goto acquired; > > > > Wait, why do we consider this as a successful acquisition? The > > vm_refcnt is 0, so this is similar situation to an earlier: Right yeah, same question? > > > > if (!refcount_add_not_zero(VMA_LOCK_OFFSET, &vma->vm_refcnt)) > > return 0; > > But this means "vma is not attached" not "we failed to lock it". Yup as vma_mark_detached() always takes the last refcount afaict. > > > IOW, the vma is not referenced, so we failed to lock it. I think the > > fix should be: > > > > if (err) { > > + if (refcount_sub_and_test(VMA_LOCK_OFFSET, &vma->vm_refcnt)) { > > + /* Oh cobblers. While we got a fatal signal, we > > + * raced with the last user. VMA is not referenced, > > + * fail to lock it. > > + */ > > + err = 0; > > Returning 0 in this situation therefore wouldn't be correct. > > AFAIU since we started with attached vma above, it's not possible that > the refcount_sub_and_test here will drop the refcnt to zero. We could > just WARN_ON_ONCE() on the result (in a way to make also the > __must_check happy) and then can return err below. Yeah we are only detaching in the case that we _know_ we are the only writer right? So nobody else should be trying to get the write lock and failing like this? So maybe the 'wrinkle' as you say should be a WARN_ON()... or a VM_WARN_ON() since this 'should never happen' (famous last words TM)? > > > + } > > rwsem_release(&vma->vmlock_dep_map, _RET_IP_); > > return err; > > } > > > > > >> + } > >> rwsem_release(&vma->vmlock_dep_map, _RET_IP_); > >> return err; > >> } > >> +acquired: > >> lock_acquired(&vma->vmlock_dep_map, _RET_IP_); > >> > >> return 1; > >> -- > >> 2.47.2 > >> > > Cheers, Lorenzo