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SubjectRe: [f2fs-dev] [syzbot] BUG: unable to handle kernel NULL pointer dereference in folio_mark_dirty
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On 2021/12/8 6:10, Jaegeuk Kim wrote:
> On 12/07, Matthew Wilcox wrote:
>> On Tue, Dec 07, 2021 at 01:39:06PM -0800, Jaegeuk Kim wrote:
>>> On 12/07, Matthew Wilcox wrote:
>>>>>> Call Trace:
>>>>>> <TASK>
>>>>>> folio_mark_dirty+0x136/0x270 mm/page-writeback.c:2639
>>>>
>>>> if (likely(mapping)) {
>>>> ...
>>>> if (folio_test_reclaim(folio))
>>>> folio_clear_reclaim(folio);
>>>> return mapping->a_ops->set_page_dirty(&folio->page);
>>>>
>>>> how do we get to a NULL ->set_page_dirty for a metadata page's
>>>> mapping->a_ops? This is definitely an f2fs expert question.
>>>
>>> I can't find anything in f2fs, since that page was got by f2fs_grab_meta_page
>>> along with grab_cache_page() that we never unlocked it.
>>>
>>> 40 struct page *f2fs_grab_meta_page(struct f2fs_sb_info *sbi, pgoff_t index)
>>> 41 {
>>> 42 struct address_space *mapping = META_MAPPING(sbi);
>>> 43 struct page *page;
>>> 44 repeat:
>>> 45 page = f2fs_grab_cache_page(mapping, index, false);
>>>
>>> -> grab_cache_page(mapping, index);
>>>
>>> 46 if (!page) {
>>> 47 cond_resched();
>>> 48 goto repeat;
>>> 49 }
>>> 50 f2fs_wait_on_page_writeback(page, META, true, true);
>>> 51 if (!PageUptodate(page))
>>> 52 SetPageUptodate(page);
>>> 53 return page;
>>> 54 }
>>>
>>>
>>> Suspecting something in folio wrt folio_mapping()?
>>>
>>> 81 bool set_page_dirty(struct page *page)
>>> 82 {
>>> 83 return folio_mark_dirty(page_folio(page));
>>> 84 }
>>
>> ... huh? How could folio_mapping() be getting this wrong?
>
> Dunno.
>
>> page_folio() does the same thing as compound_head() -- as far as I know
>> you don't use compound pages for f2fs metadata, so this basically just
>> casts the page to a struct folio.
>>
>> folio_mapping() is just like the old page_mapping() (see commit
>> 2f52578f9c64). Unless you've done something like set the swapcache
>> bit on your metadata page, it's just going to return folio->mapping
>> (ie the same as page->mapping).
>
> Hmm, I've never seen this call stack before, so simply started to suspect
> folio.

I'm afraid this is a f2fs bug... :(

folio wasn't merged at the first report time (5.14-rc2).

https://syzkaller.appspot.com/bug?extid=07ff38c9c93ca170de07

I doubt the direct reason of panic may be the same as the one of bug
reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=215231

However, I still can't figure out why meta_inode's a_ops will change
to f2fs_meta_aops.

Thanks,

>
>
> _______________________________________________
> Linux-f2fs-devel mailing list
> Linux-f2fs-devel@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel

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