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

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Date: Mon, 24 Jun 2024 11:53:55 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>, kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev, 
	lkp@...el.com, Linux Memory Management List <linux-mm@...ck.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Chengming Zhou <chengming.zhou@...ux.dev>, 
	Nhat Pham <nphamcs@...il.com>, David Hildenbrand <david@...hat.com>, 
	"Huang, Ying" <ying.huang@...el.com>, Hugh Dickins <hughd@...gle.com>, 
	Johannes Weiner <hannes@...xchg.org>, Shakeel Butt <shakeel.butt@...ux.dev>, 
	Andi Kleen <ak@...ux.intel.com>, linux-kernel@...r.kernel.org
Subject: Re: [linux-next:master] [mm] 0fa2857d23: WARNING:at_mm/page_alloc.c:#__alloc_pages_noprof

On Mon, Jun 24, 2024 at 11:50 AM Usama Arif <usamaarif642@...il.com> 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@...el.com> 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.