Date: Tue, 15 Jun 2010 16:22:19 +0200
From: Andrea Arcangeli <aarcange@redhat.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: Mel Gorman <mel@csn.ul.ie>, linux-kernel@vger.kernel.org,
       linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
       Dave Chinner <david@fromorbit.com>,
       Chris Mason <chris.mason@oracle.com>, Nick Piggin <npiggin@suse.de>,
       Rik van Riel <riel@redhat.com>
Subject: Re: [RFC PATCH 0/6] Do not call ->writepage[s] from direct reclaim
 and use a_ops->writepages() where possible
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 <20100615141122.GA27893@infradead.org>
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On Tue, Jun 15, 2010 at 10:11:22AM -0400, Christoph Hellwig wrote:
> On Tue, Jun 15, 2010 at 04:00:11PM +0200, Andrea Arcangeli wrote:
> > collecting clean cache doesn't still satisfy the allocation), during
> > allocations in direct reclaim and increase the THREAD_SIZE than doing
> > this purely for stack reasons as the VM will lose reliability if we
> 
> This basically means doubling the stack size, as you can splice together
> two extremtly stack hungry codepathes in the worst case.  Do you really
> want order 2 stack allocations?

If we were forbidden to call ->writepage just because of stack
overflow yes as I don't think it's big deal with memory compaction and
I see this as a too limiting design to allow ->writepage only in
kernel thread. ->writepage is also called by the pagecache layer,
msync etc.. not just by kswapd.

But let's defer this after we have any resemblance of hard numbers of
worst-case stack usage measured during the aforementioned workload, I
didn't read all the details as I'm quite against this design, but I
didn't see any stack usage number or any sign of stack-overflow debug
triggering. I'd suggest to measure the max stack usage first and worry
later.

And if ->writepage is a stack hog in some fs, I'd rather see
->writepage made less stack hungry (with proper warning at runtime
with debug option enabled) than vetoed. The VM itself shouldn't be a
stack hog already. I don't see a particular reason why writepage
should be so stuck hungry compared to the rest of the kernel, it just
have to do I/O, if it requires complex data structure it should
kmalloc those and stay light on stack as everybody else.

And if something I'm worried more about slab shrink than ->writepage
as that enters the vfs layer and then the lowlevel fs to collect the
dentry, inode etc...
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