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

[Yosry Ahmed]

On Mon, Jun 24, 2024 at 11:50 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote:
>
>
> 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.

I believe Matthew meant reusing the xarray used by the existing
swapcache, not adding a new one for this purpose. See my response.