Re: [PATCH] mm: remove redundant clear page when CONFIG_INIT_ON_ALLOC_DEFAULT_ON configured

[Michal Hocko <mhocko@suse.com>]

On Mon 11-09-23 13:47:03, Matthew Wilcox wrote:
> On Mon, Sep 11, 2023 at 02:12:25PM +0200, Michal Hocko wrote:
> > On Mon 11-09-23 18:49:06, zhaoyang.huang wrote:
> > > From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> > > 
> > > There will be redundant clear page within vma_alloc_zeroed_movable_folio
> > > when CONFIG_INIT_ON_ALLOC_DEFAULT_ON is on. Remove it by judging related
> > > configs.
> > 
> > Thanks for spotting this. I suspect this is a fix based on a code review
> > rather than a real performance issue, right? It is always good to
> > mention that. From a very quick look it seems that many architectures
> > just definte vma_alloc_zeroed_movable_folio to use __GFP_ZERO so they
> > are not affected by this. This means that only a subset of architectures
> > are really affected. This is an important information as well.
> > Finally I think it would be more appropriate to mention that the double
> > initialization is done when init_on_alloc is enabled rather than
> > referring to the above config option which only controls whether the
> > functionality is enabled by default.
> 
> This may well be an unsaafe change to make.  We're not just zeroing the
> page, we're calling clear_user_highpage() which tells the architecture
> which virtual address the page will be mapped at.  It could be that
> skipping the zeroing ("because the page is already zero") isn't enough;
> there will be traces of the former contents of some page in the D-cache
> for this address.

I haven't realized this difference between clear_user_highpage and
kernel_init_pages  which is used by the page allocator. Thanks for
pointing this out!

> 
> Or it might just be an optimisation.  The description of clear_user_page()
> isn't entirely clear; the port may be relying on clear_user_page()
> to have flushed the dcache aliases.
> 
> At this point, I don't think this patch is worth the risk.

Agreed! Based on that I take my ack back.

> My mind is
> changable on this, but I think we'd need buy-in from ARM, SH and Xtensa
> (who directly define clear_user_highpage()) as well as Arc, csky, ia64,
> m68k, mips, nios2, parisc, powerpc, sparc who all seem to have non-trivial
> clear_user_page() implementations.

-- 
Michal Hocko
SUSE Labs