Re: [patch 03/13] io-accounting: write accounting

From: Suleiman Souhlal
Date: Wed Dec 13 2006 - 06:03:32 EST


Suleiman Souhlal wrote:
Andrew Morton wrote:

On Wed, 13 Dec 2006 00:45:50 -0800
Suleiman Souhlal <ssouhlal@xxxxxxxxxxx> wrote:


akpm@xxxxxxxx wrote:

From: Andrew Morton <akpm@xxxxxxxx>

Accounting writes is fairly simple: whenever a process flips a page from clean
to dirty, we accuse it of having caused a write to underlying storage of
PAGE_CACHE_SIZE bytes.


On architectures where dirtying a page doesn't cause a page fault (like i386), couldn't you end up billing the wrong process (in fact, I think that even on other archituctures set_page_dirty() doesn't get called immediately in the page fault handler)?



Yes, that would be a problem in 2.6.18 and earlier.

In 2.6.19 and later, we do take a fault when transitioning a page from
pte-clean to pte-dirty. That was done to get the dirty-page accounting
right - to avoid the all-of-memory-is-dirty-but-the-kernel-doesn't-know-it
problem.


Ah yes indeed. I'm unable to keep up with all the new developments. :-(

However, if my understanding of this code is correct, it seems that the
page fault is only done for shared writable VMAs, so you still can't
rely on set_page_dirty() always being called by the process that
dirtied the page in the first place.

Am I wrong?

Yes I am.
The only I/O non-shared VMAs might cause is from swapping, and I'm not
sure if the io accounting patches actually care about that.
My confusion came from the term "shared": A VMA is considered shared
whenever MAP_SHARED is specified, even if it only has only one single
"user".

-- Suleiman
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