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Message-ID: <Pine.LNX.4.64.0803281128480.17805@schroedinger.engr.sgi.com>
Date: Fri, 28 Mar 2008 11:33:05 -0700 (PDT)
From: Christoph Lameter <clameter@....com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
cc: "Rafael J. Wysocki" <rjw@...k.pl>,
Pawel Staszewski <pstaszewski@...com.pl>,
LKML <linux-kernel@...r.kernel.org>,
Adrian Bunk <bunk@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Natalie Protasevich <protasnb@...il.com>
Subject: Re: 2.6.25-rc7-git2: Reported regressions from 2.6.24
On Thu, 27 Mar 2008, Linus Torvalds wrote:
> Totally irrelevant.
>
> The page allocation path does
>
> if (gfp_flags & __GFP_ZERO)
> prep_zero_page(page, order, gfp_flags);
>
> and that will cause a warning REGARDLESS of whether the page is a HIGHMEM
> page or not.
prep_zero_page does:
static inline void prep_zero_page(struct page *page, int order, gfp_t
gfp_flags)
{
int i;
/*
* clear_highpage() will use KM_USER0, so it's a bug to use __GFP_ZERO
* and __GFP_HIGHMEM from hard or soft interrupt context.
*/
VM_BUG_ON((gfp_flags & __GFP_HIGHMEM) && in_interrupt());
for (i = 0; i < (1 << order); i++)
clear_highpage(page + i);
}
So we forbit __GFP_HIGHMEM and in_interrupt which makes sense. The simple
forwarding of large kmallocs to the page allocator as done by SLUB / SLOB
is fine.
Then clear_highpage calls additional checking functions that have
the effect of generally forbiding zeroing in interrupt context if
CONFIG_HIGHMEM is set. This is wrong and needs to be fixed.
> And the fact is, passing in GFP_ZERO from the SLUB code is a bug
> regardless, because it unnecessarily does the dual memset().
Well that is only the fallback path of __slab_alloc which is not triggered
here and not performance sensitive. We could clear the flag there but
that is irrevelant for this issue.
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