On Mon, Aug 24, 2015 at 01:59:45AM +0200, Jesper Dangaard Brouer wrote:
> On Wed, 19 Aug 2015 12:21:45 +0300
> "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
>
> > Hugh has pointed that compound_head() call can be unsafe in some
> > context. There's one example:
> >
> [...]
>
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 0735bc0a351a..a4c4b7d07473 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
>
> [...]
> > -/*
> > - * If we access compound page synchronously such as access to
> > - * allocated page, there is no need to handle tail flag race, so we can
> > - * check tail flag directly without any synchronization primitive.
> > - */
> > -static inline struct page *compound_head_fast(struct page *page)
> > -{
> > - if (unlikely(PageTail(page)))
> > - return page->first_page;
> > - return page;
> > -}
> > -
> [...]
>
> > @@ -548,13 +508,7 @@ static inline struct page *virt_to_head_page(const void *x)
> > {
> > struct page *page = virt_to_page(x);
> >
> > - /*
> > - * We don't need to worry about synchronization of tail flag
> > - * when we call virt_to_head_page() since it is only called for
> > - * already allocated page and this page won't be freed until
> > - * this virt_to_head_page() is finished. So use _fast variant.
> > - */
> > - return compound_head_fast(page);
> > + return compound_head(page);
> > }
>
> I hope this does not slow down the SLAB/slub allocator?
> (which calls virt_to_head_page() frequently)
It should be slightly faster.
Before:
00002e90 <test_virt_to_head_page>:
2e90: 8b 15 00 00 00 00 mov 0x0,%edx
2e96: 05 00 00 00 40 add $0x40000000,%eax
2e9b: c1 e8 0c shr $0xc,%eax
2e9e: c1 e0 05 shl $0x5,%eax
2ea1: 01 d0 add %edx,%eax
2ea3: 8b 10 mov (%eax),%edx
2ea5: f6 c6 80 test $0x80,%dh
2ea8: 75 06 jne 2eb0 <test_virt_to_head_page+0x20>
2eaa: c3 ret
2eab: 90 nop
2eac: 8d 74 26 00 lea 0x0(%esi,%eiz,1),%esi
2eb0: 8b 40 1c mov 0x1c(%eax),%eax
2eb3: c3 ret
After:
00003070 <test_virt_to_head_page>:
3070: 8b 15 00 00 00 00 mov 0x0,%edx
3076: 05 00 00 00 40 add $0x40000000,%eax
307b: c1 e8 0c shr $0xc,%eax
307e: c1 e0 05 shl $0x5,%eax
3081: 01 d0 add %edx,%eax
3083: 8b 50 14 mov 0x14(%eax),%edx
3086: 8d 4a ff lea -0x1(%edx),%ecx
3089: f6 c2 01 test $0x1,%dl
308c: 0f 45 c1 cmovne %ecx,%eax
308f: c3 ret
--
Kirill A. Shutemov
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