linux-kernel - Re: [PATCH 0/3] fuse: fix accounting background requests

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Date:	Wed, 6 Feb 2013 18:12:15 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Maxim Patlasov <mpatlasov@...allels.com>
Cc:	dev@...allels.com, xemul@...allels.com,
	fuse-devel@...ts.sourceforge.net, bfoster@...hat.com,
	linux-kernel@...r.kernel.org, devel@...nvz.org
Subject: Re: [PATCH 0/3] fuse: fix accounting background requests

On Wed, Dec 26, 2012 at 1:44 PM, Maxim Patlasov <mpatlasov@...allels.com> wrote:
> Hi,
>
> The feature was added long time ago (commit 08a53cdc...) with the comment:
>
>> A task may have at most one synchronous request allocated.  So these requests
>> need not be otherwise limited.
>>
>> However the number of background requests (release, forget, asynchronous
>> reads, interrupted requests) can grow indefinitely.  This can be used by a
>> malicous user to cause FUSE to allocate arbitrary amounts of unswappable
>> kernel memory, denying service.
>>
>> For this reason add a limit for the number of background requests, and block
>> allocations of new requests until the number goes bellow the limit.
>
> However, the implementation suffers from the following problems:
>
> 1. Latency of synchronous requests. As soon as fc->num_background hits the
> limit, all allocations are blocked: both for synchronous and background
> requests. This is unnecessary - as the comment cited above states, synchronous
> requests need not be limited (by fuse). Moreover, sometimes it's very
> inconvenient. For example, a dozen of tasks aggressively writing to mmap()-ed
> area may block 'ls' for long while (>1min in my experiments).
>
> 2. Thundering herd problem. When fc->num_background falls below the limit,
> request_end() calls wake_up_all(&fc->blocked_waitq). This wakes up all waiters
> while it's not impossible that the first waiter getting new request will
> immediately put it to background increasing fc->num_background again.
> (experimenting with mmap()-ed writes I observed 2x slowdown as compared with
> fuse after applying this patch-set)
>
> The patch-set re-works fuse_get_req (and its callers) to throttle only requests
> intended for background processing. Having this done, it becomes possible to
> use exclusive wakeups in chained manner: request_end() wakes up a waiter,
> the waiter allocates new request and submits it for background processing,
> the processing ends in request_end() where another wakeup happens an so on.

Thanks.  These patches look okay.

But they don't apply to for-next.  Can you please update them?

Thanks,
Miklos

>
> Thanks,
> Maxim
>
> ---
>
> Maxim Patlasov (3):
>       fuse: make request allocations for background processing explicit
>       fuse: skip blocking on allocations of synchronous requests
>       fuse: implement exclusive wakeup for blocked_waitq
>
>
>  fs/fuse/cuse.c   |    2 +-
>  fs/fuse/dev.c    |   60 +++++++++++++++++++++++++++++++++++++++++-------------
>  fs/fuse/file.c   |    5 +++--
>  fs/fuse/fuse_i.h |    3 +++
>  fs/fuse/inode.c  |    1 +
>  5 files changed, 54 insertions(+), 17 deletions(-)
>
> --
> Signature
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