Re: [RFC][PATCH] JBD: release checkpoint journal heads through try_to_release_page when the memory is exhausted

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Hi Andrew.
Thank you for your useful comment.

Andrew Morton wrote:
> (added linux-fsdevel)
>
> On Thu, 23 Oct 2008 17:41:01 +0900
> Toshiyuki Okajima <toshi.okajima@xxxxxxxxxxxxxx> wrote:
<SNIP>
> > I managed to build a shrinker callback function for the journal_head slab cache.
> > > This code size is less than before but the logic of it seems to be more complex
> > >  than before.
> > > However, I haven't got any troubles while I am testing some easy load operations
> > > on the fixed kernel.
> > > But I think a system may hang up if concurrently several journal_head shrinker
> > > are executed.
> > > So, I will retry to build more appropriate fix.
>
> yeah, that's not very pretty either, is it?
Yes...
I realized fixing only within buffer_head or jbd needs a more complex code.

> > > Please give me comments if you have a nicer idea.
> Stepping back a bit...
>
> The basic problem is, I believe, that some client of the blockdev
> (ext3) is adding metadata to the blockdev's data structures
> (buffer_heads) but we have no means by which the blockdev code can call
> back into that client requesting that the metadata be released, yes?
Yes.

> We can fix the problem which you've identified by adding a means for
> the blockdev code (def_blk_aops.releasepage()) to call back into ext3,
> yes?
Yes.

At first, I tried to fix by using only filesystem approach.
 - ver.1: fixing in buffer_head
 - ver.2: adding shrinker of journal_head (for releasing buffer_head)
But these approaches become complex code.
So, we should fix the essence of the problem.

> If so, how do we do that?
>
> I seem to recall that there's code somewhere in the tree which does
> things like taking a copy of bdev->address_space_operations and
> reinstalling that, and overwriting selected fields, and then arranging
> somehow for the old value to be reinstalled when the client releases
> the blockdev.  That's plain nasty.
uh-huh.

I try to fix this problem again by using your approach,
blkdev_register_releasepage/blkdev_unregister_releasepage().

Best Regards,
Toshiyuki Okajima

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