linux-kernel - Re: [syzbot] [mm?] WARNING in memory

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Message-ID: <cad74ef8-3543-4fc5-a175-8fc23a88776a@redhat.com>
Date: Wed, 24 Sep 2025 19:05:54 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, Luis Chamberlain <mcgrof@...nel.org>,
 "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: syzbot <syzbot+e6367ea2fdab6ed46056@...kaller.appspotmail.com>,
 akpm@...ux-foundation.org, linmiaohe@...wei.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, nao.horiguchi@...il.com,
 syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] WARNING in memory_failure


>>
>>>
>>> What I can think of is:
>>> 0. split code always does a split to allowed minimal order,
>>>      namely max(fs_min_order, order_from_caller);
>>
>> Wouldn't max mean "allowed maximum order" ?
>>
>> I guess what you mean is "split to this order or smaller" -- min?
> 
> But LBS imposes a fs_min_order that is not 0. When a caller asks
> to split to 0, folio split code needs to use fs_min_order instead of 0.
> Thus the max.

I'd say, the point is that if someone wants to split to 0 but that is 
impossible, then we should fail :)

> 
>>
>>> 1. if split order cannot reach to order_from_caller, it just return fails,
>>>      so most of the caller will know about it;
>>
>> Yes, I think this would be the case here: if we cannot split to order-0, we can just fail right away.
>>
>>> 2. for LBS code, when it sees a split failure, it should check the resulting
>>>      folio order against fs min_order. If the orders match, it regards it as
>>>      a success.
>>>
>>> At least, most of the code does not need to be LBS aware. WDYT?
>>
>> Is my understand correct that it's either that the caller wants to
>>
>> (a) Split to order-0 -- no larger folio afterwards.
>>
>> (b) Split to smallest order possible, which might be the mapping min order.
> 
> Right. IIRC, most of callers are (a), since folio split was originally
> called by code that cannot handle THPs (now large folios). For (b),
> I actually wonder if there exists such a caller.
> 
>> If so, we could keep the interface simpler than allowing to specify arbitrary orders as request.
> 
> We might just need (a), since there is no caller of (b) in kernel, except
> split_folio_to_order() is used for testing. There might be future uses
> when kernel wants to convert from THP to mTHP, but it seems that we are
> not there yet.
> 

Even better, then maybe selected interfaces could just fail if the 
min-order contradicts with the request to split to a non-larger 
(order-0) folio.

> 
> 
> +Luis and Pankaj for their opinions on how LBS is going to use split folio
> to any order.
> 
> Hi Luis and Pankaj,
> 
> It seems that bumping split folio order from 0 to mapping_min_folio_order()
> instead of simply failing the split folio call gives surprises to some
> callers and causes issues like the one reported by this email. I cannot think
> of any situation where failing a folio split does not work. If LBS code
> wants to split, it should supply mapping_min_folio_order(), right? Does
> such caller exist?
> 
> Thanks.
> 
> 
> Best Regards,
> Yan, Zi
> 


-- 
Cheers

David / dhildenb