Messages in this thread | | | Subject | [PATCH 0/4] fuse: fix accounting background requests (v2) | From | "Maxim V. Patlasov" <> | Date | Thu, 21 Mar 2013 18:01:53 +0400 |
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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.
Changed in v2: - rebased on for-next branch of the fuse tree - fixed race when processing request begins before init-reply came
Thanks, Maxim
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Maxim V. Patlasov (4): fuse: make request allocations for background processing explicit fuse: add flag fc->uninitialized fuse: skip blocking on allocations of synchronous requests fuse: implement exclusive wakeup for blocked_waitq
fs/fuse/cuse.c | 3 ++ fs/fuse/dev.c | 69 +++++++++++++++++++++++++++++++++++++++++++----------- fs/fuse/file.c | 6 +++-- fs/fuse/fuse_i.h | 8 ++++++ fs/fuse/inode.c | 4 +++ 5 files changed, 73 insertions(+), 17 deletions(-)
-- Signature
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