Re: [PATCH] afs: Fix waiting for writeback then skipping folio

[Andrew Morton]

On Fri, 16 Jun 2023 23:43:02 +0100 David Howells <dhowells@xxxxxxxxxx> wrote:

> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> > > Commit acc8d8588cb7 converted afs_writepages_region() to write back a
> > > folio batch. The function waits for writeback to a folio, but then
> > > proceeds to the rest of the batch without trying to write that folio
> > > again. This patch fixes has it attempt to write the folio again.
> > > 
> > > This has only been compile tested.
> > 
> > This seems fairly serious?
> 
> We will try to write the again later, but sync()/fsync() might now have
> skipped it.
> 
> > From my reading, we'll fail to write out the dirty data.  Presumably
> > not easily observable, as it will get written out again later on.
> 
> As it's a network filesystem, interactions with third parties could cause
> apparent corruption.  Closing a file will flush it - but if there's a
> simultaneous op of some other kind, a bit of a flush or a sync may get missed
> and the copy visible to another user be temporarily missing that bit.
> 
> > But we're also calling afs_write_back_from_locked_folio() with an unlocked
> > folio, which might cause mayhem.
> 
> Without this patch, you mean?  There's a "continue" statement that should send
> us back to the top of the loop before we get as far as
> afs_write_back_from_locked_folio() - and then the folio_unlock() there would
> go bang.
> 

Well, what I'm really asking is the thing I ask seven times a day:

- what are the end-user visible effects of the bug

- should be fix be backported into earlier kernels