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

[Matthew Wilcox]

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@xxxxxxxxxx>
> > 
> > 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.

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.  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.