Re: [PATCH] lib/scatterlist: Fix to merge contiguous pages into the last SG properly

From: Logan Gunthorpe <logang@deltatee.com>
To: Jason Gunthorpe <jgg@nvidia.com>, Yishai Hadas <yishaih@nvidia.com>
Cc: 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
Date: Thu, 5 Jan 2023 13:23:52 -0700	[thread overview]
Message-ID: <81470f28-2bcf-b3cc-4824-6e45343dbd77@deltatee.com> (raw)
In-Reply-To: <Y7ctsw8ffhSsBZ4v@nvidia.com>

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/

Though when the patch was correct when I originally wrote it and it
looks like I merged it poorly somewhere along the line (roughly v5 of
the series) when the paddr stuff was added. Sorry about that.
The paddr stuff was messy and really hard to understand.

Anyway, Yishai's first patch looks correct to me, but I guess we need to
fix it further. For what it's worth:

Reviewed-by: Logan Gunthorpe <logang@deltatee.com>

Logan