From: David Rientjes <rientjes@google.com>
To: zhihua che <zhihua.che@gmail.com>
Cc: Christoph Lameter <cl@linux.com>,
Pekka Enberg <penberg@cs.helsinki.fi>,
linux-kernel@vger.kernel.org
Subject: Re: Fwd: Slub Allocator: Why get_order(size * MAX_OBJS_PER_PAGE) - 1 in function slab_order()?
Date: Wed, 23 Nov 2011 15:06:37 -0800 (PST) [thread overview]
Message-ID: <alpine.DEB.2.00.1111231501030.24794@chino.kir.corp.google.com> (raw)
In-Reply-To: <CABexPfHxM=NpwrrbFrTJUVZHzORpVaqAwoh1yC7d=RTfwnQWwg@mail.gmail.com>
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On Wed, 23 Nov 2011, zhihua che wrote:
> I know what you mean, that is, a slab can only store no more than
> MAX_OBJS_PER_PAGE, actually 0x7FFF, objects.
>
> But get_order(size * MAX_OBJS_PER_PAGE) already returns the order
> which reserves no_more_than size * MAX_OBJS_PER_PAGE memory. Right?
>
Yes, but it reserves too much memory if the conditional is true.
> So I think there is no need to subtract one.
>
If we didn't subtract one, then the order of a slab page would allow for
_more_ than MAX_OBJS_PER_PAGE to be allocated and that's not allowed
because of the restrictions in struct page.
Consider a page size of 4K and an object size of 8 bytes.
get_order(8 * 32767) would be 6, so that's a 4K * 2^6 = 256K slab page
without the subtraction and could allocate (256K * 1024 / 8) = 32768 which
is greater than MAX_OBJS_PER_PAGE and not allowed.
So we subtract one so the compound slab page is guaranteed to allocate
less than MAX_OBJS_PER_PAGE.
next prev parent reply other threads:[~2011-11-23 23:06 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-11-22 20:40 Slub Allocator: Why get_order(size * MAX_OBJS_PER_PAGE) - 1 in function slab_order()? zhihua che
2011-11-23 2:53 ` David Rientjes
[not found] ` <CABexPfEte8U5pVBJ=LvRKMY5=oW7F2vRjw_d9893Yy8W88hfDg@mail.gmail.com>
2011-11-23 6:59 ` Fwd: " zhihua che
2011-11-23 23:06 ` David Rientjes [this message]
[not found] ` <CABexPfFFiNf8gj9EHnnWABmcK_wCiRkhxQObUnr0_oY2Dkp+JA@mail.gmail.com>
2011-11-24 4:44 ` zhihua che
2011-11-24 4:55 ` zhihua che
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