Re: [regression] oops on heavy compilations ("kernel BUG at mm/zswap.c:1005!" and "Oops: invalid opcode: 0000")

[Matthew Wilcox <willy@infradead.org>]

On Fri, Aug 23, 2024 at 10:35:19AM -0400, Nhat Pham wrote:
> On Fri, Aug 23, 2024 at 9:13 AM Matthew Wilcox <willy@infradead.org> wrote:
> >
> >
> > That said, zswap could handle this better.  There's no need to panic the
> > entire machine over being unable to read a page from swap.  Killing just
> > the process that needed this page is sufficient.
> 
> Agree 100%. It is silly to kill the entire host for a swap read error,
> and extra silly to kill the process because we fail to writeback - for
> all we know that page might never be needed by the process again!!!
> 
> >
> > Suggested patch at end after the oops.
> >
> > @@ -1601,6 +1613,7 @@ bool zswap_load(struct folio *folio)
> >         bool swapcache = folio_test_swapcache(folio);
> >         struct xarray *tree = swap_zswap_tree(swp);
> >         struct zswap_entry *entry;
> > +       int err;
> >
> >         VM_WARN_ON_ONCE(!folio_test_locked(folio));
> >
> > @@ -1638,10 +1651,13 @@ bool zswap_load(struct folio *folio)
> >         if (!entry)
> >                 return false;
> >
> > -       if (entry->length)
> > -               zswap_decompress(entry, folio);
> > -       else
> > +       if (entry->length) {
> > +               err = zswap_decompress(entry, folio);
> > +               if (err)
> > +                       return false;
> 
> Here, if zswap decompression fails and zswap load returns false, the
> page_io logic will proceed as if zswap does not have the page and
> reads garbage from the backing device instead. This could potentially
> lead to silent data/memory corruption right? Or am I missing something
> :) Maybe we could be extra careful here and treat it as if there is a
> bio read error in the case zswap owns the page, but cannot decompress
> it?

Ah; you know more about how zswap works than I do.  So it's not a
write-through cache?  I guess we need to go a bit further then and
return an errno from zswap_load -- EIO/ENOENT/0 and handle that
appropriately.

> The rest seems solid to me :)