On Tue, Feb 25, 2025 at 11:57:27PM -0500, Johannes Weiner wrote: > On Wed, Feb 26, 2025 at 03:12:35AM +0000, Yosry Ahmed 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. > > It's analogous to what __end_swap_bio_read() does for block backends, > so it's hitchhiking on the standard swap protocol for read failures. Right, that's also how I got the idea when I did the same for large folios handling. > > The page sticks around if there are other users. It can get reclaimed, > but since it's not marked dirty, it won't get overwritten. Another > access will either find it in the swapcache and die on !uptodate; if > it was reclaimed, it will attempt another decompression. If all > references have been killed, zswap_invalidate() will finally drop it. > > Swapoff actually poisons the page table as well (unuse_pte). Right. My question was basically why don't we also poison the page table in do_swap_page() in this case. It's like that we never swapoff. This will cause subsequent fault attempts to return VM_FAULT_HWPOISON quickly without doing through the swapcache or decompression. Probably not a big deal, but shmem does it so maybe it'd be nice to do it for consistency. > > > (2) A hwpoisoned swap entry results in VM_FAULT_SIGBUS in some cases > > (e.g. shmem_fault() -> shmem_get_folio_gfp() -> shmem_swapin_folio()), > > even though we have VM_FAULT_HWPOISON. This patch falls under this > > bucket, but unfortunately we cannot tell for sure if it's a hwpoision or > > a decompression bug. > > Are you sure? Actual memory failure should replace the ptes of a > mapped shmem page with TTU_HWPOISON, which turns them into special > swap entries that trigger VM_FAULT_HWPOISON in do_swap_page(). I was looking at the shmem_fault() path. It seems like for this path we end up with VM_SIGBUS because shmem_swapin_folio() returns -EIO and not -EHWPOISON. This seems like something that can be easily fixed though, unless -EHWPOISON is not always correct for a diffrent reason. > > Anon swap distinguishes as long as the swapfile is there. Swapoff > installs poison markers, which are then handled the same in future > faults (VM_FAULT_HWPOISON): > > /* > * "Poisoned" here is meant in the very general sense of "future accesses are > * invalid", instead of referring very specifically to hardware memory errors. > * This marker is meant to represent any of various different causes of this. > * > * Note that, when encountered by the faulting logic, PTEs with this marker will > * result in VM_FAULT_HWPOISON and thus regardless trigger hardware memory error > * logic. If that's the case, maybe it's better for zswap in the future if we stop relying on not marking the folio uptodate, and instead propagate an error through swap_read_folio() to the callers to make sure we always return VM_FAULT_HWPOISON and install poison markers. The handling is a bit quirky and inconsistent, but it ultimately results in VM_SIGBUS or VM_FAULT_HWPOISON which I guess is fine for now. > */ > #define PTE_MARKER_POISONED BIT(1)
