Re: [PATCH] iomap: Handle I/O errors gracefully in page_mkwrite

[Matthew Wilcox]

On Fri, Jun 05, 2020 at 10:31:59AM +1000, Dave Chinner wrote:
> On Thu, Jun 04, 2020 at 04:50:50PM -0700, Matthew Wilcox wrote:
> > > Sure, but that's not really what I was asking: why isn't this
> > > !uptodate state caught before the page fault code calls
> > > ->page_mkwrite? The page fault code has a reference to the page,
> > > after all, and in a couple of paths it even has the page locked.
> > 
> > If there's already a PTE present, then the page fault code doesn't
> > check the uptodate bit.  Here's the path I'm looking at:
> > 
> > do_wp_page()
> >  -> vm_normal_page()
> >  -> wp_page_shared()
> >      -> do_page_mkwrite()
> > 
> > I don't see anything in there that checked Uptodate.
> 
> Yup, exactly the code I was looking at when I asked this question.
> The kernel has invalidated the contents of a page, yet we still have
> it mapped into userspace as containing valid contents, and we don't
> check it at all when userspace generates a protection fault on the
> page?

Right.  The iomap error path only clears PageUptodate.  It doesn't go
to the effort of unmapping the page from userspace, so userspace has a
read-only view of a !Uptodate page.

> > I think the iomap code is the only filesystem which clears PageUptodate
> > on errors. 
> 
> I don't think you looked very hard. A quick scan shows at least
> btrfs, f2fs, hostfs, jffs2, reiserfs, vboxfs and anything using the
> iomap path will call ClearPageUptodate() on a write IO error.

I'll give you btrfs and jffs2, but I don't think it's true for f2fs.
The only other filesystem using the iomap bufferd IO paths today
is zonefs, afaik.