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

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Date:   Thu, 5 Jan 2023 18:48:50 +0200
From:   Yishai Hadas <yishaih@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     <linux-kernel@...r.kernel.org>, <linux-block@...r.kernel.org>,
        <linux-mm@...ck.org>, <axboe@...nel.dk>, <logang@...tatee.com>,
        <hch@....de>, <alex.williamson@...hat.com>, <leonro@...dia.com>,
        <maorg@...dia.com>
Subject: Re: [PATCH] lib/scatterlist: Fix to merge contiguous pages into the
 last SG properly

On 05/01/2023 15:36, 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@...dia.com>
>> ---
>>   lib/scatterlist.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Thanks Jason

>
> 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?


Yes, I have locally some candidate patch as you asked, on top of this one.

I would like to run some extra testing on, then may send it.

Yishai