Re: [PATCH] mm/zswap: flush dcache in the compressed decompression path

[Yosry Ahmed]

On Fri, Jul 10, 2026 at 11:40 AM Joshua Hahn <joshua.hahnjy@xxxxxxxxx> wrote:
>
> On Fri, 10 Jul 2026 11:22:43 -0700 Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
>
> > On Fri, Jul 10, 2026 at 1:28 AM Song Hu <husong@xxxxxxxxxx> wrote:
> > >
> > > Both branches of zswap_decompress() write the destination folio through a
> > > kernel mapping: the incompressible branch via kmap_local_folio() and the
> > > compressed branch via the crypto scatterwalk. Yet only the incompressible
> > > branch calls flush_dcache_folio(); the compressed branch was missed. On
> > > aliasing D-cache architectures this can leave stale cache lines visible to
> > > userland after a zswap load.
> >
> > Adding Herbert here.
> >
> > Doesn't the Crypto API take care of this for the decompression path?
> > Maybe in scomp_acomp_comp_decomp()?
>
> This is what I think too. Actually if you use something like
> zstd_decompress() as well, it ends up calling:
> acomp_walk_done_dst() --> scatterwalk_done_dst()
>                       --> __scatterwalk_flush_dcache_pages()
>                       --> flush_dcache_page()
> over all the pages in the loop.
>
> Maybe I'm missing a path here that would require us to manually flush
> from zswap? But even still, I think that should be handled by the
> path that is repsonsible for doing the actual decompression, not
> zswap.
>
> What do you think?

Yeah I am hoping Herbert will help us out here.