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Date:   Thu, 5 Jan 2023 16:25:47 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Logan Gunthorpe <logang@deltatee.com>
Cc:     Yishai Hadas <yishaih@nvidia.com>, linux-kernel@vger.kernel.org,
        linux-block@vger.kernel.org, linux-mm@kvack.org, axboe@kernel.dk,
        hch@lst.de, alex.williamson@redhat.com, leonro@nvidia.com,
        maorg@nvidia.com
Subject: Re: [PATCH] lib/scatterlist: Fix to merge contiguous pages into the
 last SG properly
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On Thu, Jan 05, 2023 at 01:23:52PM -0700, Logan Gunthorpe wrote:
> 
> 
> On 2023-01-05 13:06, Jason Gunthorpe wrote:
> > On Thu, Jan 05, 2023 at 01:23:39PM +0200, Yishai Hadas wrote:
> >> When sg_alloc_append_table_from_pages() calls to pages_are_mergeable()
> >> in its 'sgt_append->prv' flow to check whether it can merge contiguous
> >> pages into the last SG, it passes the page arguments in the wrong order.
> >>
> >> The first parameter should be the next candidate page to be merged to
> >> the last page and not the opposite.
> >>
> >> The current code leads to a corrupted SG which resulted in OOPs and
> >> unexpected errors when non-contiguous pages are merged wrongly.
> >>
> >> Fix to pass the page parameters in the right order.
> >>
> >> Fixes: 1567b49d1a40 ("lib/scatterlist: add check when merging zone device pages")
> >> Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
> >> ---
> >>  lib/scatterlist.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > rdma is pretty much the only user of this API and this bug is causing
> > bad data corruption, so I'm going to take it to the rdma tree and send
> > it tomorrow.
> > 
> > Which raises the question why the original patch was done at all,
> > nothing ever inputs pgmap pages into this function?
> 
> It was done solely because you had suggested it was necessary.
> 
> https://lore.kernel.org/all/20210929224653.GZ964074@nvidia.com/

Yes, but that was when I was expecting this would work with
FOLL_LONGTERM and PUP..

Jason