On Sun, Mar 24, 2024 at 9:54 PM Barry Song <21cnbao@xxxxxxxxx> wrote: > > On Mon, Mar 25, 2024 at 10:23 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > > > On Sun, Mar 24, 2024 at 2:04 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > > > > > Zhongkun He reports data corruption when combining zswap with zram. > > > > > > The issue is the exclusive loads we're doing in zswap. They assume > > > that all reads are going into the swapcache, which can assume > > > authoritative ownership of the data and so the zswap copy can go. > > > > > > However, zram files are marked SWP_SYNCHRONOUS_IO, and faults will try > > > to bypass the swapcache. This results in an optimistic read of the > > > swap data into a page that will be dismissed if the fault fails due to > > > races. In this case, zswap mustn't drop its authoritative copy. > > > > > > Link: https://lore.kernel.org/all/CACSyD1N+dUvsu8=zV9P691B9bVq33erwOXNTmEaUbi9DrDeJzw@xxxxxxxxxxxxxx/ > > > Reported-by: Zhongkun He <hezhongkun.hzk@xxxxxxxxxxxxx> > > > Fixes: b9c91c43412f ("mm: zswap: support exclusive loads") > > > Cc: stable@xxxxxxxxxxxxxxx [6.5+] > > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> > > > Tested-by: Zhongkun He <hezhongkun.hzk@xxxxxxxxxxxxx> > > Acked-by: Barry Song <baohua@xxxxxxxxxx> > > > > > Do we also want to mention somewhere (commit log or comment) that > > keeping the entry in the tree is fine because we are still protected > > from concurrent loads/invalidations/writeback by swapcache_prepare() > > setting SWAP_HAS_CACHE or so? > > It seems that Kairui's patch comprehensively addresses the issue at hand. > Johannes's solution, on the other hand, appears to align zswap behavior > more closely with that of a traditional swap device, only releasing an entry > when the corresponding swap slot is freed, particularly in the sync-io case. It actually worked out quite well that Kairui's fix landed shortly before this bug was reported, as this fix wouldn't have been possible without it as far as I can tell. > > Johannes' patch has inspired me to consider whether zRAM could achieve > a comparable outcome by immediately releasing objects in swap cache > scenarios. When I have the opportunity, I plan to experiment with zRAM. That would be interesting. I am curious if it would be as straightforward in zram to just mark the folio as dirty in this case like zswap does, given its implementation as a block device.
