Re: [PATCH 5/9] revert "mm/z3fold.c: allow __GFP_HIGHMEM in z3fold_alloc"

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 19 May 2022 19:34:01 +0800 Miaohe Lin <linmiaohe@huawei.com> wrote:

> On 2022/5/19 15:12, Vitaly Wool wrote:
> > On Fri, Apr 29, 2022 at 8:41 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
> >>
> >> Revert commit f1549cb5ab2b ("mm/z3fold.c: allow __GFP_HIGHMEM in
> >> z3fold_alloc").
> >>
> >> z3fold can't support GFP_HIGHMEM page now. page_address is used
> >> directly at all places. Moreover, z3fold_header is on per cpu
> >> unbuddied list which could be access anytime. So we should rid
> >> the support of GFP_HIGHMEM allocation for z3fold.
> > 
> > Could you please clarify how kmem_cache is affected here?
> 
> With this code changes, kmem_cache should be unaffected. HIGHMEM is still not supported for
> kmem_cache just like before but caller ensures __GFP_HIGHMEM is not passed in now. The issue
> I want to fix here is that if z3fold page is allocated from highmem, page_address can't be
> used directly. Did I answer your question? Or don't I get your point?
> 

Yes, page_address() against a highmem page only works if that page has
been mapped into pagetables with kmap() or kmap_atomic(), and z3fold
doesn't appear to do that.

Given that other zpool_driver implementations do appear to support
highmem pages, I expect that z3fold should be taught likewise.


I didn't look very hard, but this particular patch is a bit worrisome. 
As I understand it, zswap can enable highmem:

	if (zpool_malloc_support_movable(entry->pool->zpool))
		gfp |= __GFP_HIGHMEM | __GFP_MOVABLE;

and z3fold will silently ignore the __GFP_HIGHMEM, which is OK.  But
with this patch, z3fold will now return -EINVAL, so existing setups
will start failing?