Re: [PATCHv4 06/39] thp, mm: avoid PageUnevictable on active/inactivelru lists

[Kirill A. Shutemov]

Dave Hansen wrote:
> On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> > 
> > active/inactive lru lists can contain unevicable pages (i.e. ramfs pages
> > that have been placed on the LRU lists when first allocated), but these
> > pages must not have PageUnevictable set - otherwise shrink_active_list
> > goes crazy:
> > 
> > kernel BUG at /home/space/kas/git/public/linux-next/mm/vmscan.c:1122!
> > invalid opcode: 0000 [#1] SMP
> > CPU 0
> > Pid: 293, comm: kswapd0 Not tainted 3.8.0-rc6-next-20130202+ #531
> > RIP: 0010:[<ffffffff81110478>]  [<ffffffff81110478>] isolate_lru_pages.isra.61+0x138/0x260
> > RSP: 0000:ffff8800796d9b28  EFLAGS: 00010082'
> ...
> 
> I'd much rather see a code snippet and description the BUG_ON() than a
> register and stack dump.  That line number is wrong already. ;)

Good point.

> > For lru_add_page_tail(), it means we should not set PageUnevictable()
> > for tail pages unless we're sure that it will go to LRU_UNEVICTABLE.
> > Let's just copy PG_active and PG_unevictable from head page in
> > __split_huge_page_refcount(), it will simplify lru_add_page_tail().
> > 
> > This will fix one more bug in lru_add_page_tail():
> > if page_evictable(page_tail) is false and PageLRU(page) is true, page_tail
> > will go to the same lru as page, but nobody cares to sync page_tail
> > active/inactive state with page. So we can end up with inactive page on
> > active lru.
> > The patch will fix it as well since we copy PG_active from head page.
> 
> This all seems good, and if it fixes a bug, it should really get merged
> as it stands.  Have you been actually able to trigger that bug in any
> way in practice?

I was only able to trigger it on ramfs transhuge pages split.
I doubt there's a way to reproduce it on current upstream code.

> Acked-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

Thanks!

-- 
 Kirill A. Shutemov
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