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Date:   Fri, 12 Nov 2021 09:44:41 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        linux-block@vger.kernel.org,
        Geert Uytterhoeven <geert@linux-m68k.org>
Subject: Re: [PATCH] blk-mq: setup blk_mq_alloc_data.cmd_flags after
 submit_bio_checks() is done
Message-ID: <20211112084441.GA32120@lst.de>
References: <20211112081137.406930-1-ming.lei@redhat.com> <20211112082140.GA30681@lst.de> <YY4nv5eQUTOF5Wfv@T590>
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On Fri, Nov 12, 2021 at 04:37:19PM +0800, Ming Lei wrote:
> > can only be used for reads, and no fua can be set if the preallocating
> > I/O didn't use fua, etc.
> > 
> > What are the pitfalls of just chanigng cmd_flags?
> 
> Then we need to check cmd_flags carefully, such as hctx->type has to
> be same, flush & passthrough flags has to be same, that said all
> ->cmd_flags used for allocating rqs have to be same with the following
> bio->bi_opf.
> 
> In usual cases, I guess all IOs submitted from same plug batch should be
> same type. If not, we can switch to change cmd_flags.

Jens: is this a limit fitting into your use cases?

I guess as a quick fix this rejecting different flags is probably the
best we can do for now, but I suspect we'll want to eventually relax
them.