On Fri, Dec 18, 2020 at 10:10:01PM -0800, John Hubbard wrote: > On 12/18/20 9:18 PM, Matthew Wilcox wrote: > > On Fri, Dec 18, 2020 at 10:03:16PM +0000, Matthew Wilcox wrote: > > > On Fri, Dec 18, 2020 at 04:05:31PM +0000, Matthew Wilcox wrote: > > > > A number of implementations of ->set_page_dirty check whether the page > > > > has been truncated (ie page->mapping has become NULL since entering > > > > set_page_dirty()). Several other implementations assume that they can do > > > > page->mapping->host to get to the inode. So either some implementations > > > > are doing unnecessary checks or others are vulnerable to a NULL pointer > > > > dereference if truncate() races with set_page_dirty(). > > > > > > > > I'm touching ->set_page_dirty() anyway as part of the page folio > > > > conversion. I'm thinking about passing in the mapping so there's no > > > > need to look at page->mapping. > > > > > > > > The comments on set_page_dirty() and set_page_dirty_lock() suggests > > > > there's no consistency in whether truncation is blocked or not; we're > > > > only guaranteed that the inode itself won't go away. But maybe the > > > > comments are stale. > > > > > > The comments are, I believe, not stale. Here's some syzbot > > > reports which indicate that ext4 is seeing races between set_page_dirty() > > > and truncate(): > > > > > > https://groups.google.com/g/syzkaller-lts-bugs/c/s9fHu162zhQ/m/Phnf6ucaAwAJ > > > > > > The reproducer includes calls to ftruncate(), so that would suggest > > > that's what's going on. > > > > Hmmm ... looks like __set_page_dirty_nobuffers() has a similar problem: > > > > { > > lock_page_memcg(page); > > if (!TestSetPageDirty(page)) { > > struct address_space *mapping = page_mapping(page); > > unsigned long flags; > > > > if (!mapping) { > > unlock_page_memcg(page); > > return 1; > > } > > > > xa_lock_irqsave(&mapping->i_pages, flags); > > BUG_ON(page_mapping(page) != mapping); > > > > sure, we check that the page wasn't truncated between set_page_dirty() > > and the call to TestSetPageDirty(), but we can truncate dirty pages > > with no problem. So between the call to TestSetPageDirty() and > > the call to xa_lock_irqsave(), the page can be truncated, and the > > BUG_ON should fire. > > > > I haven't been able to find any examples of this, but maybe it's just a very > > narrow race. Does anyone recognise this signature? Adding the filesystems > > which use __set_page_dirty_nobuffers() directly without extra locking. > > > That sounds like the same *kind* of failure that Jan Kara and I were > seeing on live systems[1], that led eventually to the gup-to-pup > conversion exercise. > > That crash happened due to calling set_page_dirty() on pages that had no > buffers on them [2]. And that sounds like *exactly* the same thing as > calling __set_page_dirty_nobuffers() without extra locking. So I'd > expect that it's Just Wrong To Do, for the same reasons as Jan spells > out very clearly in [1]. Interesting. It's a bit different, *but* Jan's race might be what's causing this symptom. The reason is that the backtrace contains set_page_dirty_lock() which holds the page lock. So there can't be a truncation race because truncate holds the page lock when calling ->invalidatepage. That said, the syzbot reproducer doesn't have any O_DIRECT in it either. So maybe this is some other race? > Hope that helps. > > > [1] https://www.spinics.net/lists/linux-mm/msg142700.html > > [2] which triggered this assertion: > > #define page_buffers(page) \ > ({ \ > BUG_ON(!PagePrivate(page)); \ > ((struct buffer_head *)page_private(page)); \ > }) > > > > > > $ git grep set_page_dirty.*=.*__set_page_dirty_nobuffers > > fs/9p/vfs_addr.c: .set_page_dirty = __set_page_dirty_nobuffers, > > fs/cifs/file.c: .set_page_dirty = __set_page_dirty_nobuffers, > > fs/cifs/file.c: .set_page_dirty = __set_page_dirty_nobuffers, > > fs/fuse/file.c: .set_page_dirty = __set_page_dirty_nobuffers, > > fs/hostfs/hostfs_kern.c: .set_page_dirty = __set_page_dirty_nobuffers, > > fs/jfs/jfs_metapage.c: .set_page_dirty = __set_page_dirty_nobuffers, > > fs/nfs/file.c: .set_page_dirty = __set_page_dirty_nobuffers, > > fs/ntfs/aops.c: .set_page_dirty = __set_page_dirty_nobuffers, /* Set the page dirty > > fs/orangefs/inode.c: .set_page_dirty = __set_page_dirty_nobuffers, > > fs/vboxsf/file.c: .set_page_dirty = __set_page_dirty_nobuffers, > > > > ...wow, long list of these. > > thanks, > -- > John Hubbard > NVIDIA
