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
