On Mon, Jun 08, 2026 at 04:38:46AM -0400, Michael S. Tsirkin wrote:
> In __free_pages_prepare(), when FPI_ZEROED is set the page is already
> known to be zero. We can skip kernel_init_pages() if page poisoning is
> not enabled (because poison would overwrite the zeroes).
>
> This avoids redundant zeroing work when freeing pages that are already
> known to contain all zeros.
>
> Signed-off-by: Michael S. Tsirkin <[email protected]>
> Assisted-by: Claude:claude-opus-4-6
> ---
> mm/page_alloc.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 008f1a311c40..e3a7c40c769c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1443,7 +1443,14 @@ __always_inline bool __free_pages_prepare(struct page
> *page,
> if (kasan_has_integrated_init())
> init = false;
> }
> - if (init)
> + /*
> + * Skip redundant zeroing when the page is already known-zero
> + * (FPI_ZEROED) and page poisoning did not overwrite it.
> + * When page_poisoning is enabled, kernel_poison_pages above
> + * wrote PAGE_POISON (0xAA), so we must re-zero.
> + */
Again, please stop specifying arbitrary hex values in comments, this seems
mostly 'describing what we do here'.
Maybe drop to just e.g.:
/* if poisoned or not zeroed by a virtualised host, zero now. */
or suchlike?
> + if (init && !((fpi_flags & FPI_ZEROED) &&
> + !page_poisoning_enabled_static()))
This condition is absolutely horrible, !(X && !Y), you're making life difficult
for the readers.
'if not both zeroed and not poisoned' is how that reads logically. Which is hard
to understand.
De Morgan's law gives us -> !zeroed || posioned
How about:
if (init && (!(fpi_flags & FPI_ZEROED) ||
page_poisoning_enabled_static())
Or preferably something like:
const bool poisoned = page_poisoning_enabled_static();
const bool vm_host_zeroed = fpi_flags & FPI_ZEROED;
...
if (init && (poisoned || !vm_host_zeroed))
...
?
> kernel_init_pages(page, 1 << order);
>
> /*
> --
> MST
>
Thanks, Lorenzo
