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Re: XFS status update for May 2012

To: Andreas Dilger <adilger@xxxxxxxxx>
Subject: Re: XFS status update for May 2012
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Mon, 18 Jun 2012 16:16:12 -0500
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, "linux-fsdevel@xxxxxxxxxxxxxxx Devel" <linux-fsdevel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <4FDF9998.6020205@xxxxxxxxxxx>
References: <20120618120853.GA15480@xxxxxxxxxxxxx> <AD997E9D-2C1E-4EE4-80D7-2A5C998B6E9E@xxxxxxxxx> <4FDF9998.6020205@xxxxxxxxxxx>
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On 6/18/12 4:11 PM, Eric Sandeen wrote:
> On 6/18/12 1:25 PM, Andreas Dilger wrote:
>> On 2012-06-18, at 6:08 AM, Christoph Hellwig wrote:
>>> May saw the release of Linux 3.4, including a decent sized XFS update.
>>> Remarkable XFS features in Linux 3.4 include moving over all metadata
>>> updates to use transactions, the addition of a work queue for the
>>> low-level allocator code to avoid stack overflows due to extreme stack
>>> use in the Linux VM/VFS call chain,
>>
>> This is essentially a workaround for too-small stacks in the kernel,
>> which we've had to do at times as well, by doing work in a separate
>> thread (with a new stack) and waiting for the results?  This is a
>> generic problem that any reasonably-complex filesystem will have when
>> running under memory pressure on a complex storage stack (e.g. LVM +
>> iSCSI), but causes unnecessary context switching.
>>
>> Any thoughts on a better way to handle this, or will there continue
>> to be a 4kB stack limit and hack around this with repeated kmalloc
> 
> well, 8k on x86_64 (not 4k) right?   But still...
> 
> Maybe it's still a partial hack but it's more generic - should we have
> IRQ stacks like x86 has?  (I think I'm right that that only exists
> on x86 / 32-bit) - is there any downside to that?

Maybe I'm wrong about that, and we already have IRQ stacks on x86_64 -
at least based on the kernel documentation?

-Eric

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