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

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Thu, Feb 27, 2025 at 01:16:16AM -0500, Johannes Weiner wrote:
> 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.

I know, but looking at the two, this one makes it clearer to me there
are 3 distinct cases, and the redundancy is not terrible.

So I personally prefer the latter, but I am fine either way.