Messages in this thread | | | Date | Thu, 2 Aug 2012 20:51:31 +0400 | From | Glauber Costa <> | Subject | Re: [PATCH] slub: use free_page instead of put_page for freeing kmalloc allocation |
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On 08/02/2012 08:42 PM, Johannes Weiner wrote: > On Thu, Aug 02, 2012 at 09:06:41AM -0500, Christoph Lameter wrote: >> On Thu, 2 Aug 2012, Glauber Costa wrote: >> >>> diff --git a/mm/slub.c b/mm/slub.c >>> index e517d43..9ca4e20 100644 >>> --- a/mm/slub.c >>> +++ b/mm/slub.c >>> @@ -3453,7 +3453,7 @@ void kfree(const void *x) >>> if (unlikely(!PageSlab(page))) { >>> BUG_ON(!PageCompound(page)); >>> kmemleak_free(x); >>> - put_page(page); >>> + __free_pages(page, compound_order(page)); >> >> Hmmm... put_page would have called put_compound_page(). which would have >> called the dtor function. dtor is set to __free_pages() ok which does >> mlock checks and verifies that the page is in a proper condition for >> freeing. Then it calls free_one_page(). >> >> __free_pages() decrements the refcount and then calls __free_pages_ok(). >> >> So we loose the checking and the dtor stuff with this patch. Guess that is >> ok? > > The changelog is not correct, however. People DO get pages underlying > slab objects and even free the slab objects before returning the page. > See recent fix:
Well, yes, in the sense that slab objects are page-backed.
The point is that a user of kmalloc/kfree should not treat a memory area as if it were a page, even if it is page-sized.
If it is just the Changelog you are unhappy about, I can do another submission rewording it.
> commit 5bf5f03c271907978489868a4c72aeb42b5127d2 > Author: Pravin B Shelar <pshelar@nicira.com> > Date: Tue May 29 15:06:49 2012 -0700 > > mm: fix slab->page flags corruption > > Transparent huge pages can change page->flags (PG_compound_lock) without > taking Slab lock. Since THP can not break slab pages we can safely access > compound page without taking compound lock. > > Specifically this patch fixes a race between compound_unlock() and slab > functions which perform page-flags updates. This can occur when > get_page()/put_page() is called on a page from slab.
This is just another argument not to do put_page on slab pages!
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