Re: [PATCH v2] zswap: do not crash the kernel on decompression failure

Johannes Weiner
2025-02-27T06:16:22Z

On Thu, Feb 27, 2025 at 05:44:29AM +0000, Yosry Ahmed wrote:
> On Wed, Feb 26, 2025 at 11:31:41PM -0500, Johannes Weiner wrote:
> > On Thu, Feb 27, 2025 at 01:19:31AM +0000, Yosry Ahmed wrote:
> > > On Wed, Feb 26, 2025 at 04:14:45PM -0800, Nhat Pham wrote:
> > > > if (WARN_ON_ONCE(folio_test_large(folio)))
> > > > return true;
> > > >
> > > > + entry = xa_load(tree, offset);
> > > > + if (!entry)
> > > > + return false;
> > > > +
> > >
> > > A small comment here pointing out that we are deliberatly not setting
> > > uptodate because of the failure may make things more obvious, or do you
> > > think that's not needed?
> > >
> > > > + if (!zswap_decompress(entry, folio))
> > > > + return true;
> >
> > How about an actual -ev and have this in swap_read_folio():
>
> Good idea, I was going to suggest an enum but this is simpler.
>
> >
> > ret = zswap_load(folio);
> > if (ret != -ENOENT) {
> > folio_unlock(folio);
> > goto finish;
> > }
> >
> > read from swapfile...
> >
> > Then in zswap_load(), move uptodate further up like this (I had
> > previously suggested this):
> >
> > if (!zswap_decompress(entry, folio))
> > return -EIO;
> >
> > folio_mark_uptodate(folio);
> >
> > and I think it would be clear, even without or just minimal comments.
>
> Another possibility is moving folio_mark_uptodate() back to
> swap_read_folio(), which should make things even clearer imo as the
> success/failure logic is all in one place:

That works. bdev, swapfile and zeromap set the flag in that file.

> ret = zswap_load(folio);
> if (ret != -ENOENT) {
> folio_unlock(folio);
> /* Comment about not marking uptodate */
> if (!ret)
> folio_mark_uptodate();
> goto finish;
> }

Personally, I like this one ^. The comment isn't needed IMO, as now
zswap really isn't doing anything special compared to the others.

> or we can make it crystal clear we have 3 distinct cases:
>
> ret = zswap_load(folio);
> if (!ret) {
> folio_unlock(folio);
> folio_mark_uptodate();
> goto finish;
> } else if (ret != -ENOENT) {
> /* Comment about not marking uptodate */
> folio_unlock(folio);
> goto finish;
> }

This seems unnecessarily repetetive.