On Sat, Mar 23, 2024 at 8:38 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > On Fri, Mar 22, 2024 at 9:40 AM <chengming.zhou@xxxxxxxxx> wrote: > > > > From: Chengming Zhou <chengming.zhou@xxxxxxxxx> > > > > There is a report of data corruption caused by double swapin, which is > > only possible in the skip swapcache path on SWP_SYNCHRONOUS_IO backends. > > > > The root cause is that zswap is not like other "normal" swap backends, > > it won't keep the copy of data after the first time of swapin. So if I don't quite understand this, so once we load a page from zswap, zswap will free it even though do_swap_page might not set it to PTE? shouldn't zswap free the memory after notify_free just like zram? > > the folio in the first time of swapin can't be installed in the pagetable > > successfully and we just free it directly. Then in the second time of > > swapin, we can't find anything in zswap and read wrong data from swapfile, > > so this data corruption problem happened. > > > > We can fix it by always adding the folio into swapcache if we know the > > pinned swap entry can be found in zswap, so it won't get freed even though > > it can't be installed successfully in the first time of swapin. > > A concurrent faulting thread could have already checked the swapcache > before we add the folio to it, right? In this case, that thread will > go ahead and call swap_read_folio() anyway. > > Also, I suspect the zswap lookup might hurt performance. Would it be > better to add the folio back to zswap upon failure? This should be > detectable by checking if the folio is dirty as I mentioned in the bug > report thread. I don't like the idea either as sync-io is the fast path for zram etc. or, can we use the way of zram to free compressed data? >
