Hi Ming,

On Fri, Nov 12, 2021 at 1:47 PM Ming Lei [off-list ref] wrote:

On Fri, Nov 12, 2021 at 09:44:41AM +0100, Christoph Hellwig wrote:

quoted

On Fri, Nov 12, 2021 at 04:37:19PM +0800, Ming Lei wrote:

quoted

quoted

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.

rw mixed workload will be affected, so I think we need to switch to
change cmd_flags, how about the following patch?

From 9ab77b7adee768272944c20b7cffc8abdb85a35b Mon Sep 17 00:00:00 2001
From: Ming Lei <redacted>
Date: Fri, 12 Nov 2021 08:14:38 +0800
Subject: [PATCH] blk-mq: fix filesystem I/O request allocation

submit_bio_checks() may update bio->bi_opf, so we have to initialize
blk_mq_alloc_data.cmd_flags with bio->bi_opf after submit_bio_checks()
returns when allocating new request.

In case of using cached request, fallback to allocate new request if
cached rq isn't compatible with the incoming bio, otherwise change
rq->cmd_flags with incoming bio->bi_opf.

Fixes: 900e080752025f00 ("block: move queue enter logic into blk_mq_submit_bio()")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <redacted>

Thanks, this alternative patch seems to work fine, too.
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds