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

[Nhat Pham <nphamcs@gmail.com>]

On Tue, Feb 25, 2025 at 7:12 PM Yosry Ahmed <yosry.ahmed@linux.dev> wrote:
>
> On Tue, Feb 25, 2025 at 01:32:00PM -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.
>
> Some relevant observations/questions, but not really actionable for this
> patch, perhaps some future work, or more likely some incoherent
> illogical thoughts :
>
> (1) It seems like not making the folio uptodate will cause shmem faults
> to mark the swap entry as hwpoisoned, but I don't see similar handling
> for do_swap_page(). So it seems like even if we SIGBUS the process,
> other processes mapping the same page could follow in the same
> footsteps.

poisoned, I think? It's the weird SWP_PTE_MARKER thing.

[...]

>
>
> (3) If we run into a decompression failure, should we free the
> underlying memory from zsmalloc? I don't know. On one hand if we free it
> zsmalloc may start using it for more compressed objects. OTOH, I don't
> think proper hwpoison handling will kick in until the page is freed.
> Maybe we should tell zsmalloc to drop this page entirely and mark
> objects within it as invalid? Probably not worth the hassle but
> something to think about.

This might be a fun follow up :) I guess my question is - is there a
chance that we might recover in the future?

For example, can memory (hardware) failure somehow recover, or the
decompression algorithm somehow fix itself? I suppose not?

If that is the case, one thing we can do is just free the zsmalloc
slot, then mark the zswap entry as corrupted somehow. We can even
invalidate the zswap entry altogether, and install a (shared)
ZSWAP_CORRUPT_ENTRY. Future readers can check for this and exit if
they encounter a corrupted entry?

It's not common enough (lol hopefully) for me to optimize right away,
but I can get on with it if there are actual data of this happening
IRL/in product :)ion