On Mon, Mar 25, 2024 at 2:46 AM Chengming Zhou <chengming.zhou@xxxxxxxxx> wrote: > > On 2024/3/25 17:40, Yosry Ahmed wrote: > > On Mon, Mar 25, 2024 at 2:22 AM Chengming Zhou <chengming.zhou@xxxxxxxxx> wrote: > >> > >> On 2024/3/25 16:38, Yosry Ahmed wrote: > >>> On Mon, Mar 25, 2024 at 12:33 AM Chengming Zhou > >>> <chengming.zhou@xxxxxxxxx> wrote: > >>>> > >>>> On 2024/3/25 15:06, Yosry Ahmed wrote: > >>>>> 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. > >>>>> > >>>> > >>>> This makes me wonder who is responsible for marking folio dirty in this swapcache > >>>> bypass case? Should we call folio_mark_dirty() after the swap_read_folio()? > >>> > >>> In shrink_folio_list(), we try to add anonymous folios to the > >>> swapcache if they are not there before checking if they are dirty. > >>> add_to_swap() calls folio_mark_dirty(), so this should take care of > >> > >> Right, thanks for your clarification, so should be no problem here. > >> Although it was a fix just for MADV_FREE case. > >> > >>> it. There is an interesting comment there though. It says that PTE > >>> should be dirty, so unmapping the folio should have already marked it > >>> as dirty by the time we are adding it to the swapcache, except for the > >>> MADV_FREE case. > >> > >> It seems to say the folio will be dirtied when unmap later, supposing the > >> PTE is dirty. > > > > Oh yeah it could mean that the folio will be dirted later. > > > >> > >>> > >>> However, I think we actually unmap the folio after we add it to the > >>> swapcache in shrink_folio_list(). Also, I don't immediately see why > >>> the PTE would be dirty. In do_swap_page(), making the PTE dirty seems > >> > >> If all anon pages on LRU list are faulted by write, it should be true. > >> We could just use the zero page if faulted by read, right? > > > > This applies for the initial fault that creates the folio, but this is > > a swap fault. It could be a read fault and in that case we still need > > to make the folio dirty because it's not in the swapcache and we need > > to write it out if it's reclaimed, right? > > Yes, IMHO I think it should be marked as dirty here. > > But it should be no problem with that unconditional folio_mark_dirty() > in add_to_swap(). Not sure if there are other issues. I don't believe there are any issues now. Dirtying the folio in add_to_swap() was introduced before SWP_SYNCHRONOUS_IO, so I guess things have always worked. I think we should update the comment there though to mention that dirtying the folio is also needed for this case (not just MADV_FREE), or dirty the PTE during the fault. Otherwise, if someone is making MADV_FREE changes they could end up breaking SWP_SYNCHRONOUS_IO faults. Adding Minchan here in case he can confirm that we in fact rely on add_to_swap()->folio_mark_dirty() for SWP_SYNCHRONOUS_IO to work as intended.
