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

Yosry Ahmed
2025-03-01T02:20:20Z

On Fri, Feb 28, 2025 at 06:08:16PM -0800, Nhat Pham wrote:
> On Thu, Feb 27, 2025 at 1:55 PM Yosry Ahmed <yosry.ahmed@linux.dev> wrote:
> >
> > On Thu, Feb 27, 2025 at 01:46:29PM -0800, Nhat Pham wrote:
> > > On Wed, Feb 26, 2025 at 5:19 PM Yosry Ahmed <yosry.ahmed@linux.dev> wrote:
> > > >
> > > > On Wed, Feb 26, 2025 at 04:14:45PM -0800, Nhat Pham wrote:
> > > > > Currently, we crash the kernel when a decompression failure occurs in
> > > > > zswap (either because of memory corruption, or a bug in the compression
> > > > > algorithm). This is overkill. We should only SIGBUS the unfortunate
> > > > > process asking for the zswap entry on zswap load, and skip the corrupted
> > > > > entry in zswap writeback. The former is accomplished by returning true
> > > > > from zswap_load(), indicating that zswap owns the swapped out content,
> > > > > but without flagging the folio as up-to-date. The process trying to swap
> > > > > in the page will check for the uptodate folio flag and SIGBUS (see
> > > > > do_swap_page() in mm/memory.c for more details).
> > > >
> > > > We should call out the extra xarray walks and their perf impact (if
> > > > any).
> > >
> > > Lemme throw this in a quick and dirty test. I doubt there's any
> > > impact, but since I'm reworking this patch for a third version anyway
> > > might as well.
> >
> > It's likely everything is cache hot and the impact is minimal, but let's
> > do the due diligence.
> >
>
> Yeah I ran some kernel building tests for 5 times, and found basically
> no difference:
>
> With the new scheme:
>
> real: mean: 125.1s, stdev: 0.12s
>
> user: mean: 3265.23s, stdev: 9.62s
>
> sys: mean: 2156.41s, stdev: 13.98s
>
>
> The old scheme:
>
> real: mean: 125.78s, stdev: 0.45s
>
> user: mean: 3287.18s, stdev: 5.95s
>
> sys: mean: 2177.08s, stdev: 26.52s
>
>
> Honestly, eyeballing the results, the mean difference is probably
> smaller than between-run variance. :)

Thanks for checking, that's good. Let's include that in the commit
message too.