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Re: [PATCH 0/2] xfs: fix some new memory allocation failures

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 0/2] xfs: fix some new memory allocation failures
From: Mark Tinguely <tinguely@xxxxxxx>
Date: Tue, 03 Sep 2013 15:46:24 -0500
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20130903200401.GF23571@dastard>
References: <1378119180-31380-1-git-send-email-david@xxxxxxxxxxxxx> <5224C4E9.8080604@xxxxxxx> <20130902222004.GI12779@dastard> <5225DF07.4080509@xxxxxxx> <20130903200401.GF23571@dastard>
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On 09/03/13 15:04, Dave Chinner wrote:
On Tue, Sep 03, 2013 at 08:07:19AM -0500, Mark Tinguely wrote:
On 09/02/13 17:20, Dave Chinner wrote:
On Mon, Sep 02, 2013 at 12:03:37PM -0500, Mark Tinguely wrote:
On 09/02/13 05:52, Dave Chinner wrote:
Hi folks,

These failures are a result of order-4 allocations being done on v5
filesystems to support the large ACL count xattrs. The first patch
puts out usual falbback to vmalloc workaround in place. The second
patch factors all the places we now have this fallback-to-vmalloc
and makes it transparent to the callers.

Cheers,

Dave.

Thanks for clean up. Broken record time: Do we really need order
allocation in the filesystem? Esp in xfs_ioctl.c.

I don't understand your question. Are you asking why we need high
order allocation?

Cheers,

Dave.

In patch 2, why not drop the physically contiguous allocation
attempt and just do the virtually contiguous allocation?

Because:

        a) virtual memory space is extremely limited on some
        platforms - we regularly get people reporting that they've
        exhausted vmalloc space on 32 bit systems.
        b) when there is free contiguous memory, allocating that
        contiguous memory is much faster than allocating
        virtual memory.
        c) virtual memory access is slower than physical memory
        access and it puts pressure on the page tables.

IOWs, we want to avoid allocating virtual memory if at all possible.

Cheers,

Dave.

Ummm, It is all virtual memory it all runs through page tables. The MMU works on virtual addresses.

It appears Linux has a special range of kernel virtual memory for the physical contiguous allocations and range for sparse memory allocation.

XFS does not need the physical space that backs the kernel virtual address to be contiguous - other parts of the kernel do. Why put pressure on the drivers that need order allocations when we do not need it?

--Mark.

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