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

[Usama Arif]

On 24/06/2024 21:33, Matthew Wilcox wrote:
> 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).

I was going for the most space-efficient and simplest data structure, 
which is bitmap. I believe xarray is either pointer or integer between 0 
and LONG_MAX? We could convert the individual bits into integer and 
store them, and have another function to extract the integer stored in 
xarray to a bit, but I think thats basically a separate bitmap_xarray 
API (which would probably take more space than a traditional bitmap API, 
and I dont want to make this series dependent on something like that), 
so I would prefer to use bitmap.