On 2023/6/28 15:22, Ming Lei wrote: > On Wed, Jun 28, 2023 at 12:55:49PM +0800, Chengming Zhou wrote: >> On 2023/6/28 12:13, Ming Lei wrote: >>> On Tue, Jun 27, 2023 at 08:08:52PM +0800, chengming.zhou@xxxxxxxxx wrote: >>>> From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx> >>>> >>>> The flush state machine use a double list to link all inflight >>>> flush_data requests, to avoid issuing separate post-flushes for >>>> these flush_data requests which shared PREFLUSH. >>>> >>>> So we can't reuse rq->queuelist, this is why we need rq->flush.list >>>> >>>> In preparation of the next patch that reuse rq->queuelist for flush >>>> state machine, we change the double linked list to a u64 counter, >>>> which count all inflight flush_data requests. >>>> >>>> This is ok since we only need to know if there is any inflight >>>> flush_data request, so a u64 counter is good. The only problem I can >>>> think of is that u64 counter may overflow, which should be unlikely happen. >>> >>> It won't overflow, q->nr_requests is 'unsigned long', which should have >>> been limited to one more reasonable value, such as 2 * BLK_MQ_MAX_DEPTH, so >>> u16 should be big enough in theory. >> >> Ah, right. q->nr_requests is 'unsigned long' and q->queue_depth is 'unsigned int', >> so 'unsigned long' counter here won't overflow. > > Not like q->nr_requests, q->queue_depth usually means the whole queue's depth, > which may cover all hw queue's depth. And it is only used by scsi, but it > should be held in "unsigned short" too. > >> >> Should I change it to smaller 'unsigned short' or just leave it as 'unsigned long' ? >> (Now the size of struct blk_flush_queue is exactly 64 bytes) > > You have to limit q->nr_requests first, which may need a bit more work for avoiding > compiling warning or sort of thing. And 64k is big enough for holding per-queue > scheduler request. > > Once it is done, it is fine to define this counter as 'unsigned short'. > Ok, I looked around these related code, found it maybe subtle to me for now. So I'd better just leave it 'unsigned long' here. :) Thanks.
