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 21:50:21 +0300
From: Usama Arif <usamaarif642@...il.com>
To: Matthew Wilcox <willy@...radead.org>, Yosry Ahmed <yosryahmed@...gle.com>
Cc: 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 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.