From: "H. Peter Anvin" <hpa@zytor.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: Why do we set _PAGE_DIRTY for page tables?
Date: Tue, 22 Apr 2014 19:51:22 -0700 [thread overview]
Message-ID: <53572AAA.4070207@zytor.com> (raw)
In-Reply-To: <CA+55aFwsPs12_57YEBHdb4ti1BXSuDX_RPSf6S4JSRLGK_2X7Q@mail.gmail.com>
On 04/22/2014 07:48 PM, Linus Torvalds wrote:
> On Tue, Apr 22, 2014 at 4:35 PM, H. Peter Anvin <hpa@zytor.com> wrote:
>> I just noticed this:
>>
>> #define _PAGE_TABLE (_PAGE_PRESENT | _PAGE_RW | _PAGE_USER | \
>> _PAGE_ACCESSED | _PAGE_DIRTY)
>> #define _KERNPG_TABLE (_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED | \
>> _PAGE_DIRTY)
>>
>> Is there a reason we set _PAGE_DIRTY for page tables? It has no
>> function, but doesn't do any harm either (the dirty bit is ignored for
>> page tables)... it just looks funny to me.
>
> I think it just got copied, and at least the A bit does matter even in
> page tables (well, it gets updated, I don't know how much that
> "matters"). So the fact that D is ignored is actually the odd man out.
>
Yes, not setting the A bit means the hardware will take an assist to set
the bit for us, which is a waste of time if we don't care about it. The
D bit is the one which made me wonder; I thought either it was just copy
& paste, or that it got set to make it more analogous with large pages.
-hpa
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WARNING: multiple messages have this Message-ID (diff)
From: "H. Peter Anvin" <hpa@zytor.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: Why do we set _PAGE_DIRTY for page tables?
Date: Tue, 22 Apr 2014 19:51:22 -0700 [thread overview]
Message-ID: <53572AAA.4070207@zytor.com> (raw)
In-Reply-To: <CA+55aFwsPs12_57YEBHdb4ti1BXSuDX_RPSf6S4JSRLGK_2X7Q@mail.gmail.com>
On 04/22/2014 07:48 PM, Linus Torvalds wrote:
> On Tue, Apr 22, 2014 at 4:35 PM, H. Peter Anvin <hpa@zytor.com> wrote:
>> I just noticed this:
>>
>> #define _PAGE_TABLE (_PAGE_PRESENT | _PAGE_RW | _PAGE_USER | \
>> _PAGE_ACCESSED | _PAGE_DIRTY)
>> #define _KERNPG_TABLE (_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED | \
>> _PAGE_DIRTY)
>>
>> Is there a reason we set _PAGE_DIRTY for page tables? It has no
>> function, but doesn't do any harm either (the dirty bit is ignored for
>> page tables)... it just looks funny to me.
>
> I think it just got copied, and at least the A bit does matter even in
> page tables (well, it gets updated, I don't know how much that
> "matters"). So the fact that D is ignored is actually the odd man out.
>
Yes, not setting the A bit means the hardware will take an assist to set
the bit for us, which is a waste of time if we don't care about it. The
D bit is the one which made me wonder; I thought either it was just copy
& paste, or that it got set to make it more analogous with large pages.
-hpa
next prev parent reply other threads:[~2014-04-23 2:51 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-04-22 23:35 Why do we set _PAGE_DIRTY for page tables? H. Peter Anvin
2014-04-22 23:35 ` H. Peter Anvin
2014-04-23 2:48 ` Linus Torvalds
2014-04-23 2:48 ` Linus Torvalds
2014-04-23 2:51 ` H. Peter Anvin [this message]
2014-04-23 2:51 ` H. Peter Anvin
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