Re: [RFC PATCH v1 15/57] stackdepot: Remove PAGE_SIZE compile-time constant assumption

Vlastimil Babka
2024-11-14T11:57:45Z

On 10/14/24 12:58, Ryan Roberts wrote:
> To prepare for supporting boot-time page size selection, refactor code
> to remove assumptions about PAGE_SIZE being compile-time constant. Code
> intended to be equivalent when compile-time page size is active.
>
> "union handle_parts" previously calculated the number of bits required
> for its pool index and offset members based on PAGE_SHIFT. This is
> problematic for boot-time page size builds because the actual page size
> isn't known until boot-time.
>
> We could use PAGE_SHIFT_MAX in calculating the worst case offset bits,
> but bits would be wasted that could be used for pool index when
> PAGE_SIZE is set smaller than MAX, the end result being that stack depot
> can address less memory than it should.
>
> To avoid needing to dynamically define the offset and index bit widths,
> let's instead fix the pool size and derive the order at runtime based on
> the PAGE_SIZE. This means that the fields' widths can remain static,
> with the down side being slightly increased risk of failing to allocate
> the large folio.
>
> This only affects boot-time page size builds. compile-time page size
> builds will still always allocate order-2 folios.
>
> Additionally, wrap global variables that are initialized with PAGE_SIZE
> derived values using DEFINE_GLOBAL_PAGE_SIZE_VAR() so their
> initialization can be deferred for boot-time page size builds.

This is done for pool_offset but given it's initialized by DEPOT_POOL_SIZE,
it doesn't look derived from PAGE_SIZE?

>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>

Other than that,
Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>
> ***NOTE***
> Any confused maintainers may want to read the cover note here for context:
> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/
>
> include/linux/stackdepot.h | 6 +++---
> lib/stackdepot.c | 6 +++---
> 2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/stackdepot.h b/include/linux/stackdepot.h
> index e9ec32fb97d4a..ac877a4e90406 100644
> --- a/include/linux/stackdepot.h
> +++ b/include/linux/stackdepot.h
> @@ -32,10 +32,10 @@ typedef u32 depot_stack_handle_t;
>
> #define DEPOT_HANDLE_BITS (sizeof(depot_stack_handle_t) * 8)
>
> -#define DEPOT_POOL_ORDER 2 /* Pool size order, 4 pages */
> -#define DEPOT_POOL_SIZE (1LL << (PAGE_SHIFT + DEPOT_POOL_ORDER))
> +#define DEPOT_POOL_ORDER 2 /* Pool size order, 4 pages of PAGE_SIZE_MAX */
> +#define DEPOT_POOL_SIZE (1LL << (PAGE_SHIFT_MAX + DEPOT_POOL_ORDER))
> #define DEPOT_STACK_ALIGN 4
> -#define DEPOT_OFFSET_BITS (DEPOT_POOL_ORDER + PAGE_SHIFT - DEPOT_STACK_ALIGN)
> +#define DEPOT_OFFSET_BITS (DEPOT_POOL_ORDER + PAGE_SHIFT_MAX - DEPOT_STACK_ALIGN)
> #define DEPOT_POOL_INDEX_BITS (DEPOT_HANDLE_BITS - DEPOT_OFFSET_BITS - \
> STACK_DEPOT_EXTRA_BITS)
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 5ed34cc963fc3..974351f0e9e3c 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -68,7 +68,7 @@ static void *new_pool;
> /* Number of pools in stack_pools. */
> static int pools_num;
> /* Offset to the unused space in the currently used pool. */
> -static size_t pool_offset = DEPOT_POOL_SIZE;
> +static DEFINE_GLOBAL_PAGE_SIZE_VAR(size_t, pool_offset, DEPOT_POOL_SIZE);
> /* Freelist of stack records within stack_pools. */
> static LIST_HEAD(free_stacks);
> /* The lock must be held when performing pool or freelist modifications. */
> @@ -625,7 +625,7 @@ depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
> */
> if (unlikely(can_alloc && !READ_ONCE(new_pool))) {
> page = alloc_pages(gfp_nested_mask(alloc_flags),
> - DEPOT_POOL_ORDER);
> + get_order(DEPOT_POOL_SIZE));
> if (page)
> prealloc = page_address(page);
> }
> @@ -663,7 +663,7 @@ depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
> exit:
> if (prealloc) {
> /* Stack depot didn't use this memory, free it. */
> - free_pages((unsigned long)prealloc, DEPOT_POOL_ORDER);
> + free_pages((unsigned long)prealloc, get_order(DEPOT_POOL_SIZE));
> }
> if (found)
> handle = found->handle.handle;