Re: [RFC PATCH] mm: add folio in swapcache if swapin from zswap

[Barry Song]

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?

>