On Wed, Sep 14, 2022 at 03:33:50PM +0900, Hyeonggon Yoo wrote: > On Fri, Sep 09, 2022 at 11:16:51PM +0200, Vlastimil Babka wrote: > > On 9/9/22 16:32, Hyeonggon Yoo wrote: > > > On Fri, Sep 09, 2022 at 03:44:19PM +0200, Vlastimil Babka wrote: > > >> On 9/9/22 13:05, Hyeonggon Yoo wrote: > > >> >> ----8<---- > > >> >> From d6f9fbb33b908eb8162cc1f6ce7f7c970d0f285f Mon Sep 17 00:00:00 2001 > > >> >> From: Vlastimil Babka <vbabka@xxxxxxx> > > >> >> Date: Fri, 9 Sep 2022 12:03:10 +0200 > > >> >> Subject: [PATCH 2/3] mm/migrate: make isolate_movable_page() skip slab pages > > >> >> > > >> >> In the next commit we want to rearrange struct slab fields to allow a > > >> >> larger rcu_head. Afterwards, the page->mapping field will overlap > > >> >> with SLUB's "struct list_head slab_list", where the value of prev > > >> >> pointer can become LIST_POISON2, which is 0x122 + POISON_POINTER_DELTA. > > >> >> Unfortunately the bit 1 being set can confuse PageMovable() to be a > > >> >> false positive and cause a GPF as reported by lkp [1]. > > >> >> > > >> >> To fix this, make isolate_movable_page() skip pages with the PageSlab > > >> >> flag set. This is a bit tricky as we need to add memory barriers to SLAB > > >> >> and SLUB's page allocation and freeing, and their counterparts to > > >> >> isolate_movable_page(). > > >> > > > >> > Hello, I just took a quick grasp, > > >> > Is this approach okay with folio_test_anon()? > > >> > > >> Not if used on a completely random page as compaction scanners can, but > > >> relies on those being first tested for PageLRU or coming from a page table > > >> lookup etc. > > >> Not ideal huh. Well I could improve also by switching 'next' and 'slabs' > > >> field and relying on the fact that the value of LIST_POISON2 doesn't include > > >> 0x1, just 0x2. > > > > > > What about swapping counters and freelist? > > > freelist should be always aligned. > > > > Great suggestion, thanks! > > > > Had to deal with SLAB too as there was list_head.prev also aliasing > > page->mapping. Wanted to use freelist as well, but turns out it's not > > aligned, so had to use s_mem instead. > > > > The patch that isolate_movable_page() skip slab pages was thus dropped. The > > result is in slab.git below and if nothing blows up, will restore it to -next > > > > https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/log/?h=for-6.1/fit_rcu_head > > I realized that there is also relevant comment in > include/linux/mm_types.h: > > > 62 * SLUB uses cmpxchg_double() to atomically update its freelist and counters. > > 63 * That requires that freelist & counters in struct slab be adjacent and > > 64 * double-word aligned. Because struct slab currently just reinterprets the > > 65 * bits of struct page, we align all struct pages to double-word boundaries, > > 66 * and ensure that 'freelist' is aligned within struct slab. > > 67 */ > > Also we may add a comment, > something like this? > > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -79,6 +79,9 @@ struct page { > * WARNING: bit 0 of the first word is used for PageTail(). That > * means the other users of this union MUST NOT use the bit to > * avoid collision and false-positive PageTail(). > + * > + * WARNING: lower two bits of third word is used for PAGE_MAPPING_FLAGS. > + * using those bits can lead compaction code to general protection fault. I'm really not comfortable with adding that documentation. I feel the compaction code should be fixed.
