Re: [linux-next:master] [mm] 0fa2857d23: WARNING:at_mm/page_alloc.c:#__alloc_pages_noprof

[Matthew Wilcox]

On Mon, Jun 24, 2024 at 05:05:56AM -0700, Yosry Ahmed wrote:
> On Mon, Jun 24, 2024 at 1:49 AM kernel test robot <oliver.sang@xxxxxxxxx> wrote:
> > kernel test robot noticed "WARNING:at_mm/page_alloc.c:#__alloc_pages_noprof" on:
> >
> > commit: 0fa2857d23aa170e5e28d13c467b303b0065aad8 ("mm: store zero pages to be swapped out in a bitmap")
> > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
> 
> This is coming from WARN_ON_ONCE_GFP(order > MAX_PAGE_ORDER, gfp), and
> is triggered by the new bitmap_zalloc() call in the swapon path. For a
> sufficiently large swapfile, bitmap_zalloc() (which uses kmalloc()
> under the hood) cannot be used to allocate the bitmap.

Do we need to use a bitmap?

We could place a special entry in the swapcache instead (there's
XA_ZERO_ENTRY already defined, and if we need a different entry that's
not XA_ZERO_ENTRY, there's room for a few hundred more special entries).