Re: [PATCH 3/3] mm, page_alloc: reduce static keys in prep_new_page()

[Alexander Potapenko]

On Tue, Oct 27, 2020 at 2:32 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 10/27/20 12:05 PM, Vlastimil Babka wrote:
> > On 10/27/20 10:10 AM, David Hildenbrand wrote:
> >> On 26.10.20 18:33, Vlastimil Babka wrote:
> >>> prep_new_page() will always zero a new page (regardless of __GFP_ZERO) when
> >>> init_on_alloc is enabled, but will also always skip zeroing if the page was
> >>> already zeroed on free by init_on_free or page poisoning.
> >>>
> >>> The latter check implemented by free_pages_prezeroed() can involve two
> >>> different static keys. As prep_new_page() is really a hot path, let's introduce
> >>> a single static key free_pages_not_prezeroed for this purpose and initialize it
> >>> in init_mem_debugging().
> >>
> >> Is this actually observable in practice? This smells like
> >> micro-optimization to me.
> >>
> >> Also, I thought the whole reason for static keys is to have basically no
> >> overhead at runtime, so I wonder if replacing two static key checks by a
> >> single one actually makes *some* difference.
> >
> > You're right, the difference seems to be just a single NOP. The static key
> > infrastructure seems to be working really well.
> > (At least the asm inspection made me realize that kernel_poison_pages() is
> > called unconditionally and the static key is checked inside, not inline so I'll
> > be amending patch 2...)
> >
> > Initially I thought I would be reducing 3 keys to 1 in this patch, but I got the
> > code wrong. So unless others think it's a readability improvements, we can drop
> > this patch.

I agree with David that replacing two static keys with one is probably
a micro-optimization.
Also, if someone is enabling both init_on_alloc and init_on_free, they
are already paying so much that no one is going to notice an extra
static key.

> > Or we can also reconsider this whole optimization. If the point is to be
> > paranoid and enable both init_on_free and init_on_alloc, should we trust that
> > nobody wrote something after the clearing on free via use-after-free? :) Kees/Alex?

I think we must trust the kernel to not overwrite zeroed pages.
After all, this could theoretically happen at any time, not only while
the memory chunk is freed.

> More thoughts...
>
> PAGE_POISONING_NO_SANITY skips the check on "unpoisoning" whether poison was
> corrupted
> PAGE_POISONING_ZERO uses zero instead of 0xAA as poison pattern
>
> the point of enabling both of these seems to be moot now that init_on_free
> exists, as that zeroes pages that are being freed, without checking on alloc
> that they are still zeroed.
>
> What if only one is enabled?
> - PAGE_POISONING_NO_SANITY without PAGE_POISONING_ZERO - we poison with the 0xAA
> pattern but nobody checks it, so does it give us anything over init_on_free
> writing zeroes? I don't think so?
>
> - PAGE_POISONING_ZERO without PAGE_POISONING_NO_SANITY - we use zeroes (like
> init_on_free) but also check that it wasn't corrupted. We save some time on
> writing zeroes again on alloc, but the check is still expensive. And writing
> 0xAA would possibly detect more corruptions than writing zero (a stray write of
> NULL is more likely to happen than of 0xAA?).
>
> So my conclusion:
> - We can remove PAGE_POISONING_NO_SANITY because it only makes sense with
> PAGE_POISONING_ZERO, and we can use init_on_free instead

Agreed.

> - We can also probably remove PAGE_POISONING_ZERO, because if we want to do the
> unpoisoning sanity check, then we also most likely want the 0xAA pattern and not
> zero.

Agreed.
It might also make sense to somehow merge page poisoning and
init_on_free together and have one config dimension instead of two
(providing something similar to the
INIT_STACK_NONE/INIT_STACK_ALL_ZERO/INIT_STACK_ALL_PATTERN configs)

> Thoughts?
>


-- 
Alexander Potapenko
Software Engineer

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