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

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Date: Fri, 5 Jan 2024 16:45:13 +0100
From: David Hildenbrand <david@...hat.com>
To: "Yin, Fengwei" <fengwei.yin@...el.com>,
 Ryan Roberts <ryan.roberts@....com>,
 syzbot <syzbot+50ef73537bbc393a25bb@...kaller.appspotmail.com>,
 akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 syzkaller-bugs@...glegroups.com, Matthew Wilcox <willy@...radead.org>
Subject: Re: [syzbot] [mm?] WARNING in __folio_rmap_sanity_checks

>> vm_normal_page() works on these mappings, so we'd also have to skip rmap
>> code when unmapping these pages etc. Maybe that's the whole reason we
>> have the rmap handling here: to not special-case the unmap path.
> 
> vm_insert_page() will set VM_MIXEDMAP and vm_normal_page() will skip
> the page if CONFIG_ARCH_HAS_PTE_SPECIAL is enabled (it's enabled for
> x86).  So the unmap path will skip these kind of folios?

I think we run into the
	if (likely(!pte_special(pte)))
		goto check_pfn;

first and return these folios. That also matches the comment of 
vm_normal_page: "VM_MIXEDMAP mappings can likewise contain memory with 
or without ... _all_ pages with a struct page (that is, those where 
pfn_valid is true) are refcounted and considered normal pages by the VM."

-- 
Cheers,

David / dhildenb