Re: [syzbot] [mm?] BUG: Bad page map (8)

[Hillf Danton]

On Fri, 19 Jul 2024 13:21:30 +0200 David Hildenbrand <david@xxxxxxxxxx>
> On 19.07.24 00:51, syzbot wrote:
> > Hello,
> > 
> > syzbot found the following issue on:
> > 
> > HEAD commit:    4d145e3f830b Merge tag 'i2c-for-6.10-rc8' of git://git.ker..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=11321495980000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=6b5a15443200e31
> > dashboard link: https://syzkaller.appspot.com/bug?extid=ec4b7d82bb051330f15a
> > compiler:       aarch64-linux-gnu-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> > userspace arch: arm64
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=113e054e980000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1366ab85980000
> > 
> 
> The reproducer involves udmabuf. I suspect it has to do with it.
> 
> But I'm curius, does the reproducer not trigger before 4d145e3f830b on 
> mainliny?
> 
> Viveks changes are not upstream yet, but I can only speculate that we 
> have some issue similar to the one we had with hugetlb: udmabuf doing 
> things with memfd/shmem pages that it shouldn't do, because it doesn't 
> "own" these pages.
> 
> "udmabuf: Use vmf_insert_pfn and VM_PFNMAP for handling mmap" might help.

	cpu1				cpu2
	---				---
	evict()				find folio2 in page cache
	truncate_inode_folio()
	truncate_cleanup_folio();
	  // unmap folio2 from mmA
	  unmap_mapping_folio(folio2);
					mmap folio2 to mmB
	filemap_remove_folio(folio2);


If the window exists for mapping folio to userspace while indoe is evicted,
is this report false positive?