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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 06:28:13PM +0000, Matthew Wilcox wrote: > On Wed, Nov 26, 2025 at 06:06:53PM +0000, Lorenzo Stoakes wrote: > > 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... :) > > It's only "impossible" currently due to some fairly esoteric reasoning. > As far as _this_ function is concerned, it's entirely possible. > I don't want to leave this trap for the next person who calls > __vma_enter_locked(TASK_KILLABLE). Calls __vma_enter_locked(TASK_KILLABLE) _when detaching_, otherwise refcount will always be >0. So we're only looking at us changing vma_mark_detached() to use TASK_KILLABLE. As this is such a subtle corner case I still think it warrants a warning. Or at least a VM_WARN_ON_ONCE(1). A killable detacher is, as Vlasta points out, kind of an unwise thing to do anyway right? > > > > + /* > > > + * 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() > > I think this falls into the "saying what you're doing, not why > you're doing it" trap. Whereas my comment is at a higher level -- > there's a race where both exit conditions are true at the same time. > The rcuwait_wait_event() picked one option, but we would rather resolve > the race in the opposite direction. I find your comment unclear, and I think it's too succinct. I was trying to provide the most succinct-yet-still-clear example, but if you prefer higher level you're going to need more detail here. It assumes you 'just know' that: - refcount_sub_and_test(VMA_LOCK_OFFSET, &vma->vm_refcnt) means unlock - err can only be set due to a fatal signal in a non-uninterruptible task mode - spurious readers can cause an incremented reference count - that a race can exist between a spuriously raised reference count and the previous reference count check between read above and refcount subtract here - a reference count of 0 means detached - err = 0 means we are treating this VMA as detached resolving this race 'in favour of' the VMA being detached. Let's get some of this information in here please. > > > And: > > > > /* Shouldn't be possible - VMA entirely detached, so treat it as such. */ > > > > Before err = 0? > > Again though, saying it's "not possible" relies on knowing all the > callers of this function behave a particular way, and there's no > guarantee they'll continue to do so. Again I think we'd be better off with at least a VM_WARN_ON_ONCE() given this is a rather obscure corner case. Thanks, Lorenzo