On Wed, Dec 26, 2012 at 1:44 PM, Maxim Patlasov <mpatla...@parallels.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
