Re: [RFC][PATCH] proc: export more page flags in /proc/kpageflags

On Tue, Apr 14, 2009 at 03:54:40PM +0900, KOSAKI Motohiro wrote:
Hi

There are two use cases here:

First what is useful for the administrator as a general abstraction.
And what is useful for the kernel hacker for debugging.

The kernel hacker wants everything even if it's subject to change,
the administrator wants a higher level abstraction they can make
sense of and that doesn't change too often.

I think there's a case for both usages, but perhaps they
should be separated (in a public and a internal interface perhaps?)

My comments below are about abstractions for the first case.



On Tue, Apr 14, 2009 at 12:37:10PM +0800, KOSAKI Motohiro wrote:
Export the following page flags in /proc/kpageflags,
just in case they will be useful to someone:

- PG_swapcache
- PG_swapbacked
- PG_mappedtodisk
- PG_reserved

PG_reserved should be exported as PG_KERNEL or somesuch.

OK.

rest problem is, how do we write document this.
PG_reserved have multiple meanings...


- PG_private
- PG_private_2
- PG_owner_priv_1

- PG_head
- PG_tail
- PG_compound

I would combine these three into a pseudo "large page" flag.

Ah good idea.


- PG_unevictable
- PG_mlocked

- PG_poison

PG_poison is also useful to export. But since it depends on my
patchkit I will pull a patch for that into the HWPOISON series.

Yes, I agree.

- PG_unevictable
- PG_mlocked

this 9 flags shouldn't exported.
I can't imazine administrator use what purpose those flags.

I think an abstraced "PG_pinned" or somesuch flag that combines
page lock, unevictable, mlocked would be useful for the administrator.

PG_unevictable and PG_mlocked have a bit delicate meaning.
it gurantee the page isn't evicted. but mlock(2) don't gurantee
turn page on PG_mlocked. some race prevent it.

I'm afraid administrator confuse it. but if someone can write good
document, my worriness will vanished.





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