On 4 October 2017 at 09:53, Linus Walleij [off-list ref] wrote:

On Tue, Oct 3, 2017 at 8:30 AM, Adrian Hunter [off-list ref] wrote:

quoted

On 02/10/17 17:09, Linus Walleij wrote:

quoted

On Sun, Oct 1, 2017 at 12:57 PM, Tetsuo Handa
[off-list ref] wrote:

quoted

quoted

quoted

I inserted u-SD card, only to realize that it is not detected as it
should be. And dmesg indeed reveals:

Tetsuo asked me to report this to linux-mm.

But 2^4 is 16 pages, IIRC that can't be expected to work reliably, and
thus this sounds like MMC bug, not mm bug.

I'm not sure I fully understand this error message:
"worker/2:1: page allocation failure: order:4"

What I guess from context is that the mmc_init_request()
call is failing to allocate 16 pages, meaning for 4K pages
64KB which is the typical bounce buffer.

This is what the code has always allocated as bounce buffer,
but it used to happen upfront, when probing the MMC block layer,
rather than when allocating the requests.

That is not exactly right.  As I already wrote, the memory allocation used
to be optional but became mandatory with:

  commit 304419d8a7e9204c5d19b704467b814df8c8f5b1
  Author: Linus Walleij [off-list ref]
  Date:   Thu May 18 11:29:32 2017 +0200

      mmc: core: Allocate per-request data using the block layer core

Yes you are right, it used to look like this, with the bounce buffer
hiding behind a Kconfig symbol:

#ifdef CONFIG_MMC_BLOCK_BOUNCE
    if (host->max_segs == 1) {
        unsigned int bouncesz;

        bouncesz = MMC_QUEUE_BOUNCESZ;

        if (bouncesz > host->max_req_size)
            bouncesz = host->max_req_size;
        if (bouncesz > host->max_seg_size)
            bouncesz = host->max_seg_size;
        if (bouncesz > (host->max_blk_count * 512))
            bouncesz = host->max_blk_count * 512;

        if (bouncesz > 512 &&
            mmc_queue_alloc_bounce_bufs(mq, bouncesz)) {
            blk_queue_bounce_limit(mq->queue, BLK_BOUNCE_ANY);
            blk_queue_max_hw_sectors(mq->queue, bouncesz / 512);
            blk_queue_max_segments(mq->queue, bouncesz / 512);
            blk_queue_max_segment_size(mq->queue, bouncesz);

            ret = mmc_queue_alloc_bounce_sgs(mq, bouncesz);
            if (ret)
                goto cleanup_queue;
            bounce = true;
        }
    }
#endif

I recently concluded that I find no evidence whatsoever that anyone
turned this symbol on. Actually. (Checked defconfigs and distro configs.)
The option was just sitting there unused.
This code was never exercised except by some people who turned it
on on their custom kernels in the past. It's in practice dead code.

My patch started to allocate and use bounce buffers for all hosts
with max_segs == 1, unless specifically flagged NOT to use bounce
buffers.

That wasn't smart, I should have just deleted them. Mea culpa.

So that is why I asked Ulf to simply put the patch deleting the bounce
buffers that noone is using to fixes, and it should fix this problem.

Adrian, Linus,

Thanks for looking into the problem! I am queuing up the patch
deleting bounce buffers for fixes asap!

Kind regards
Uffe

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