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

[Joshua Hahn]

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?
Joshua