Re: [PATCH v4] page_io: zswap: do not crash the kernel on decompression failure

[Johannes Weiner]

On Thu, Mar 06, 2025 at 12:50:10PM -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.
> 
> See [1] for a recent upstream discussion about this.
> 
> The zswap writeback case is relatively straightforward to fix. For the
> zswap_load() case, we change the return behavior:
> 
> * Return 0 on success.
> * Return -ENOENT (with the folio locked) if zswap does not own the
>   swapped out content.
> * Return -EIO if zswap owns the swapped out content, but encounters a
>   decompression failure for some reasons. The folio will be unlocked,
>   but not be marked up-to-date, which will eventually cause the process
>   requesting the page to SIGBUS (see the handling of not-up-to-date
>   folio in do_swap_page() in mm/memory.c), without crashing the kernel.
> * Return -EINVAL if we encounter a large folio, as large folio should
>   not be swapped in while zswap is being used. Similar to the -EIO case,
>   we also unlock the folio but do not mark it as up-to-date to SIGBUS
>   the faulting process.
> 
> As a side effect, we require one extra zswap tree traversal in the load
> and writeback paths. Quick benchmarking on a kernel build test shows no
> performance 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
> 
> [1]: https://lore.kernel.org/all/ZsiLElTykamcYZ6J@xxxxxxxxxxxxxxxxxxxx/
> 
> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Suggested-by: Yosry Ahmed <yosry.ahmed@xxxxxxxxx>
> Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>