Re: [PATCH] virtio_pci_modern: Use GFP_ATOMIC with spin_lock_irqsave held in virtqueue_exec_admin_cmd()

["Jinhui Guo" <guojinhui.liam@bytedance.com>]

On Mon, Apr 13, 2026 at 10:17:59 +0100, David Laight wrote:
> Or do the allocate before acquiring the lock (and free it not used
> in the error path).

Hi David,

Thanks for the suggestion.

Pre-allocating the memory outside the lock is indeed a good practice,
but unfortunately it doesn't work in this specific virtqueue context.

The kmalloc() in question is not happening at the virtqueue_exec_admin_cmd()
level. Instead, it is deeply embedded inside virtqueue_add_sgs()
(specifically, in functions like alloc_indirect_split() or
virtqueue_add_indirect_packed()) to allocate indirect descriptors when
multiple SG elements are provided.

As a caller, we have no mechanism to pre-allocate this indirect descriptor
memory and pass it down to virtqueue_add_sgs(). Furthermore, virtqueue_add_sgs()
needs to atomically check the queue's num_free status, allocate the indirect
table if necessary, and update the queue pointers. All these operations
must be protected by admin_vq->lock to prevent concurrent admin command
submissions from corrupting the virtqueue state.

Therefore, allocating before acquiring the lock isn't feasible here, and
replacing GFP_KERNEL with GFP_ATOMIC (with a proper sleepable retry upon
failure) seems to be the more viable fix.

Does this make sense?

Thanks,
Jinhui