Re: [PATCH 5.10/5.15 v2 0/1 RFC] mm/truncate: fix WARNING in ext4_set_page_dirty()

[Matthew Wilcox]

On Mon, Jan 29, 2024 at 10:11:24AM +0100, Jan Kara wrote:
> On Thu 25-01-24 14:06:58, Matthew Wilcox wrote:
> > On Thu, Jan 25, 2024 at 01:09:46PM +0000, Roman Smirnov wrote:
> > > Syzkaller reports warning in ext4_set_page_dirty() in 5.10 and 5.15
> > > stable releases. It happens because invalidate_inode_page() frees pages
> > > that are needed for the system. To fix this we need to add additional
> > > checks to the function. page_mapped() checks if a page exists in the 
> > > page tables, but this is not enough. The page can be used in other places:
> > > https://elixir.bootlin.com/linux/v6.8-rc1/source/include/linux/page_ref.h#L71
> > > 
> > > Kernel outputs an error line related to direct I/O:
> > > https://syzkaller.appspot.com/text?tag=CrashLog&x=14ab52dac80000
> > 
> > OK, this is making a lot more sense.
> > 
> > The invalidate_inode_page() path (after the page_mapped check) calls
> > try_to_release_page() which strips the buffers from the page.
> > __remove_mapping() tries to freeze the page and presuambly fails.
> 
> Yep, likely.
> 
> > ext4 is checking there are still buffer heads attached to the page.
> > I'm not sure why it's doing that; it's legitimate to strip the
> > bufferheads from a page and then reattach them later (if they're
> > attached to a dirty page, they are created dirty).
> 
> Well, we really need to track dirtiness on per fs-block basis in ext4
> (which makes a difference when blocksize < page size). For example for
> delayed block allocation we reserve exactly as many blocks as we need
> (which need not be all the blocks in the page e.g. when writing just one
> block in the middle of a large hole). So when all buffers would be marked
> as dirty we would overrun our reservation. Hence at the moment of dirtying
> we really need buffers to be attached to the page and stay there until the
> page is written back.

Thanks for the clear explanation!

Isn't the correct place to ensure that this is true in
ext4_release_folio()?  I think all paths to remove buffer_heads from a
folio go through ext4_release_folio() and so it can be prohibited here
if the folio is part of a delalloc extent?

I worry that the proposed fix here cuts off only one path to hitting
this WARN_ON and we need a more comprehensive fix.