Re: [mm/sl[au]b] 3c4cafa313: canonical_address#:#[##]

Matthew Wilcox
2022-09-14T07:42:46Z

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@suse.cz>
> > >> >> 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.