Re: [PATCH v3] page_io: zswap: do not crash the kernel on decompression failure

[Johannes Weiner]

On Mon, Mar 03, 2025 at 10:34:46PM +0000, Yosry Ahmed wrote:
> On Mon, Mar 03, 2025 at 04:55:24PM -0500, Johannes Weiner wrote:
> > pr_alert_ratelimited("Decompression error from zswap (%d:%lu %s %u->%d)\n",
> > 		     swptype, swpoffset, name, clen, dlen);
> 
> Yeah this looks much more concise. It's a bit harder to parser the dmesg
> as you have to cross check the code, but hopefully this is something
> that people rarely have to do.
> 
> I don't feel strongly about adding a helper in this case, unless we want
> to add local variables (like Johannes did above), in which case a helper
> would be a good way to hide them.

pr_alert_ratelimited("Decompression error from zswap (%d:%lu %s %u->%d)\n",
		swp_type(entry->swpentry), swp_offset(entry->swpentry),
		entry->pool->tfm_name, entry->length, acomp_ctx->req->dlen);

Seriously, this does not warrant another function.

It's also valuable to keep warnings inside the problem context instead
of socking them away somewhere. It makes it clear that decompression
failure is a serious situation. We also expect this to trigger almost
never and it won't be tested routinely, so the best chance to fight
bitrot is to keep all those derefs close by. Imagine if this triggers
and the data is misleading or it crashes the system because some rules
around entry, acomp_ctx, the pool or whatever changed. Or if the work
involved in decompression changed and this is incomplete/unhelpful.