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owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Nov 26, 2025 at 07:49:13AM -0800, Suren Baghdasaryan wrote: > On Wed, Nov 26, 2025 at 7:20 AM Lorenzo Stoakes > wrote: > > > > On Wed, Nov 26, 2025 at 03:05:44PM +0000, Matthew Wilcox wrote: > > > On Wed, Nov 26, 2025 at 03:36:46PM +0100, Vlastimil Babka wrote: > > > > On 11/26/25 5:28 AM, Suren Baghdasaryan wrote: > > > > >> Suren, Liam, Vlastimil, Lorenzo ... none of you spotted this bug. > > > > > > > > > > Doh! This is embarassing... > > > > > > > > Hand-rolled synchronization primitives are wonderful, aren't they? > > > > > > That's why I liked the original approach of just using rwsems. I > > > mst admit to having not paid attention to this recently so I don't > > > know what motivated the change. > > That made it simpler to add SLAB_TYPESAFE_BY_RCU for vm_area_structs > which improved the performance of allocating these structs during > calls like mmap and fork. Hmm, we doubled down :) Hope the wins were worth the complexity, but obviously the horse has bolted and I should have asked more about it at the time... > > > > > > > > > Wait, why do we consider this as a successful acquisition? The > > > > > vm_refcnt is 0, so this is similar situation to an earlier: > > > > > > > > > > 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". > > > > > > > > > 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. > > > > > > But how do we know that we started with an attached VMA? Maybe the VMA > > > was in the process of being detached and still has readers? > > > > So we're talking about: > > > > vma_mark_deteched() > > -> refcount_dec_and_test() [ ref count is zero ] > > -> __vma_enter_locked() > > > > (meanwhile...) > > > > -> reader attempts to read > > -> optimistic check doesn't successfully find write locked VMA > > -> __refcount_inc_not_zero_limited_acqure() somehow doesn't notice 0 refcount and increments > > (??? how) > > > > (back to vma_mark_attached() -> __vma_enter_locked()) > > > > -> refcount_add_not_zero() returns true > > > > [ process gets fatal signal ] > > > > -> rcuwait_wait_event() errors out > > -> oopsies need to do something, maybe [VM_]WARN_ON() not right? > > > > Correct me if the above is wrong. > > > > I mean is any of this actually possible...? > > > > Seems dubious. But I guess right now we assume it _is_ possible. What a mess! > > > > (Again I wonder why we made our lives so difficult here) > > > > Anyway even if we are midway through a detach, the detach is ostensibly waiting > > for the readers to go away, and our reader is about to go away anyway, but the > > process has a fatal signal so do we even care? > > > > I actually wonder if a WARN_ON() is warranted to see if this even ever > > happens... > > > > OK just going to reattach... my head which just exploded from the above :P > > We are overthinking this. Vlastimil is right. If we entered with an Watch Vlastimil frame this... ;) > attached VMA (which is the case due to this check: > https://elixir.bootlin.com/linux/v6.18-rc7/source/mm/mmap_lock.c#L60) > then the only function that can detach the VMA from under us is > vma_mark_detached() but that function can't race with We hold the mmap/VMA write lock yes, but as per the comment in vma_mark_detached() + above list readers can spuriously increment the refcount (ostensibly) as per: /* Wait until vma is detached with no readers. */ So doesn't that mean we can hit the issue Matthew raised? Otherwise there would be no need to do this dance in vma_mark_detached() anyway? Maybe I'm missing something. > __vma_enter_locked() because both functions are required to hold > mmap_write_lock. vma_mark_detached() has an explicit > vma_assert_write_locked(vma) check (which implies mmap_write_lock) and > vma_start_write() calls __is_vma_write_locked() which does > mmap_assert_write_locked(vma->vm_mm). We can add a comment here or an > mmap_assert_write_locked(vma->vm_mm); at the beginning of the > __vma_enter_locked() to make that obvious. > > > Cheers, Lorenzo > Thanks, Lorenzo