On Sun, Nov 06, 2022 at 11:03:53PM +0900, Hyeonggon Yoo wrote:
> For now, only SLAB uses _mapcount field as a number of active objects in
> a slab, and other slab allocators do not use it. As 16 bits are enough
> for that, use remaining 16 bits of _mapcount as page_type even when
> SLAB is used. And then move PG_slab flag to page_type.
>
> As suggested by Matthew, store number of active objects in negative
> form and use helper when accessing or modifying it.
>
> Note that page_type is always placed in upper 16 bits of _mapcount to
> avoid confusing normal _mapcount as page_type. As underflow (actually
> I mean, yeah, overflow) is not a concern anymore, use more lower bits.
>
> Add more folio helpers for PAGE_TYPE_OPS() not to break existing
> slab implementations.
>
> Remove PG_slab check from PAGE_FLAGS_CHECK_AT_FREE. buddy will still
> check if _mapcount is properly set at free.
>
> Exclude PG_slab from hwpoison and show_page_flags() for now.
>
> Note that with this patch, page_mapped() and folio_mapped() always return
> false for slab page.
>
...
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 779a426d2cab..9494f47c4cee 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1145,7 +1145,6 @@ static int me_huge_page(struct page_state *ps, struct page *p)
> #define mlock (1UL << PG_mlocked)
> #define lru (1UL << PG_lru)
> #define head (1UL << PG_head)
> -#define slab (1UL << PG_slab)
> #define reserved (1UL << PG_reserved)
>
> static struct page_state error_states[] = {
> @@ -1155,13 +1154,6 @@ static struct page_state error_states[] = {
> * PG_buddy pages only make a small fraction of all free pages.
> */
>
> - /*
> - * Could in theory check if slab page is free or if we can drop
> - * currently unused objects without touching them. But just
> - * treat it as standard kernel for now.
> - */
> - { slab, slab, MF_MSG_SLAB, me_kernel },
> -
Hi Hyeonggon,
Actually the above part is dead code now and it's harmless to remove this.
identify_page_state() is never called when handling memory error event
on a slab page because HWPoisonHandlable() returns false for it.
If you remove it, a few other lines using MF_MSG_SLAB will be unneccessary,
so could you remove them too?
As for testing, you can test this case for example like below:
- install page-types (available in tools/vm/page-types.c),
- show the list of slab pages by "page-types -b slab -Nl" command
(the first column is PFNs) and choose one PFNs as target,
- call "page-types -a <PFN> -X" (requires hwpoison-inject module to be loaded)
In my testing server, dmesg shows like below:
[598746.497805] Injecting memory failure at pfn 0x16295b
[598746.499208] Memory failure: 0x16295b: unhandlable page.
[598746.500570] Memory failure: 0x16295b: recovery action for unknown page: Ignored
And your patchset seems not to affect this behavior.
Thanks,
Naoya Horiguchi
