Re: [PATCH RFC 0/2] mm/page_alloc: free_pcppages_bulk safeguard

[Chris Li <chrisl@kernel.org>]

Hi Kemeng,

On Mon, Aug 21, 2023 at 6:27 PM Kemeng Shi <shikemeng@huaweicloud.com> wrote:
> >
> Agreed. We assume pcp->count is protected by pcp->lock. Instead of make code
> work in case pcp->count could be changed without lock held, it's more reasonble
> to modify pcp->count with pcp->lock held in BPF program.

The lock is holded when pcp->count is modified. It is going through
the kernel page
allocation API. The issue is nest memory allocation inside spin_lock()
introduced by BPF.

The execution sequence is like this:

       count = min(pcp->count, count);

        /* Ensure requested pindex is drained first. */
        pindex = pindex - 1;
        bpf_injected_spin_lock_irqsave {
                 alloc_page();
                 original spin_lock_irqsave(&zone->lock, flags) ;
        }

Chris





Chris