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

From: Jason Gunthorpe <jgg@nvidia.com>
To: Yishai Hadas <yishaih@nvidia.com>
Cc: linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
	linux-mm@kvack.org, axboe@kernel.dk, logang@deltatee.com,
	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 09:36:30 -0400	[thread overview]
Message-ID: <Y7bSXpw40dgWlRYn@nvidia.com> (raw)
In-Reply-To: <20230105112339.107969-1-yishaih@nvidia.com>

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(-)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Also, I'm looking more closely at '156 and this is not right either:

-               unsigned long paddr =
-                       (page_to_pfn(sg_page(sgt_append->prv)) * PAGE_SIZE +
-                        sgt_append->prv->offset + sgt_append->prv->length) /
-                       PAGE_SIZE;
-
-               while (n_pages && page_to_pfn(pages[0]) == paddr) {
+               last_pg = sg_page(sgt_append->prv);
+               while (n_pages && pages_are_mergeable(last_pg, pages[0])) {

This change will break things like multi-page combining, sub page
scenarios and maybe more.

The contiguity test here has to be done a phys, it should go back to
struct page to check if the pgmap is OK.

Can you fix it as well?

Thanks,
Jason