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

[Maxim V. Patlasov]

02/06/2013 09:12 PM, Miklos Szeredi ÐÐÑÐÑ:
> On Wed, Dec 26, 2012 at 1:44 PM, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> 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?

Sorry for long delay. I'll send updated patches soon.

Thanks,
Maxim

> 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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