* Why do we set _PAGE_DIRTY for page tables?
@ 2014-04-22 23:35 ` H. Peter Anvin
0 siblings, 0 replies; 6+ messages in thread
From: H. Peter Anvin @ 2014-04-22 23:35 UTC (permalink / raw)
To: Ingo Molnar, Thomas Gleixner, Linux Kernel Mailing List,
Linus Torvalds, linux-mm@kvack.org
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.
-hpa
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^ permalink raw reply [flat|nested] 6+ messages in thread* Why do we set _PAGE_DIRTY for page tables?
@ 2014-04-22 23:35 ` H. Peter Anvin
0 siblings, 0 replies; 6+ messages in thread
From: H. Peter Anvin @ 2014-04-22 23:35 UTC (permalink / raw)
To: Ingo Molnar, Thomas Gleixner, Linux Kernel Mailing List,
Linus Torvalds, linux-mm@kvack.org
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.
-hpa
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: Why do we set _PAGE_DIRTY for page tables?
2014-04-22 23:35 ` H. Peter Anvin
@ 2014-04-23 2:48 ` Linus Torvalds
-1 siblings, 0 replies; 6+ messages in thread
From: Linus Torvalds @ 2014-04-23 2:48 UTC (permalink / raw)
To: H. Peter Anvin
Cc: Ingo Molnar, Thomas Gleixner, Linux Kernel Mailing List,
linux-mm@kvack.org
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.
Linus
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^ permalink raw reply [flat|nested] 6+ messages in thread* Re: Why do we set _PAGE_DIRTY for page tables?
@ 2014-04-23 2:48 ` Linus Torvalds
0 siblings, 0 replies; 6+ messages in thread
From: Linus Torvalds @ 2014-04-23 2:48 UTC (permalink / raw)
To: H. Peter Anvin
Cc: Ingo Molnar, Thomas Gleixner, Linux Kernel Mailing List,
linux-mm@kvack.org
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.
Linus
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: Why do we set _PAGE_DIRTY for page tables?
2014-04-23 2:48 ` Linus Torvalds
@ 2014-04-23 2:51 ` H. Peter Anvin
-1 siblings, 0 replies; 6+ messages in thread
From: H. Peter Anvin @ 2014-04-23 2:51 UTC (permalink / raw)
To: Linus Torvalds
Cc: Ingo Molnar, Thomas Gleixner, Linux Kernel Mailing List,
linux-mm@kvack.org
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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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Why do we set _PAGE_DIRTY for page tables?
@ 2014-04-23 2:51 ` H. Peter Anvin
0 siblings, 0 replies; 6+ messages in thread
From: H. Peter Anvin @ 2014-04-23 2:51 UTC (permalink / raw)
To: Linus Torvalds
Cc: Ingo Molnar, Thomas Gleixner, Linux Kernel Mailing List,
linux-mm@kvack.org
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
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2014-04-23 2:51 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz follow: Atom feed
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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
2014-04-23 2:51 ` H. Peter Anvin
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