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SubjectRe: [PATCH v2] block: bio_map_user_iov should not be limited to BIO_MAX_PAGES
On 18/04/19 11:29, Ming Lei wrote:
> On Thu, Apr 18, 2019 at 10:42:21AM +0200, Paolo Bonzini wrote:
>> On 18/04/19 04:19, Ming Lei wrote:
>>> Hi Paolo,
>>> On Wed, Apr 17, 2019 at 01:52:07PM +0200, Paolo Bonzini wrote:
>>>> Because bio_kmalloc uses inline iovecs, the limit on the number of entries
>>>> is not BIO_MAX_PAGES but rather UIO_MAXIOV, which indeed is already checked
>>>> in bio_kmalloc. This could cause SG_IO requests to be truncated and the HBA
>>>> to report a DMA overrun.
>>> BIO_MAX_PAGES only limits the single bio's max vector number, if one bio
>>> can't hold all user space request, new bio will be allocated and appended
>>> to the passthrough request if queue limits aren't reached.
>> Stupid question: where? I don't see any place starting at
>> blk_rq_map_user_iov (and then __blk_rq_map_user_iov->bio_map_user_iov)
>> that would allocate a second bio. The only bio_kmalloc in that path is
>> the one I'm patching.
> Each bio is created inside __blk_rq_map_user_iov() which is run inside
> a loop, and the created bio is added to request via blk_rq_append_bio(),
> see the following code:

Uff, I can't read apparently. :( This is the commit that introduced it:

commit 4d6af73d9e43f78651a43ee4c5ad221107ac8365
Author: Christoph Hellwig <>
Date: Wed Mar 2 18:07:14 2016 +0100

block: support large requests in blk_rq_map_user_iov

This patch adds support for larger requests in blk_rq_map_user_iov by
allowing it to build multiple bios for a request. This functionality
used to exist for the non-vectored blk_rq_map_user in the past, and
this patch reuses the existing functionality for it on the unmap side,
which stuck around. Thanks to the iov_iter API supporting multiple
bios is fairly trivial, as we can just iterate the iov until we've
consumed the whole iov_iter.

Signed-off-by: Christoph Hellwig <>
Reported-by: Jeff Lien <>
Tested-by: Jeff Lien <>
Reviewed-by: Keith Busch <>
Signed-off-by: Jens Axboe <>


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