linux-kernel - Re: [PATCH rdma-next v4 2/3] lib/scatterlist: Fix wrong update of orig

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Date:   Wed, 25 Aug 2021 07:59:27 +0300
From:   Maor Gottlieb <maorg@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     <dledford@...hat.com>, <hch@...radead.org>, <leonro@...dia.com>,
        <aelior@...vell.com>, <daniel@...ll.ch>, <airlied@...ux.ie>,
        <dennis.dalessandro@...nelisnetworks.com>,
        <dri-devel@...ts.freedesktop.org>,
        <intel-gfx@...ts.freedesktop.org>, <jani.nikula@...ux.intel.com>,
        <joonas.lahtinen@...ux.intel.com>, <linux-kernel@...r.kernel.org>,
        <linux-rdma@...r.kernel.org>, <maarten.lankhorst@...ux.intel.com>,
        <mripard@...nel.org>, <mkalderon@...vell.com>,
        <mike.marciniszyn@...nelisnetworks.com>,
        <mustafa.ismail@...el.com>, <rodrigo.vivi@...el.com>,
        <sroland@...are.com>, <shiraz.saleem@...el.com>,
        <tzimmermann@...e.de>, <linux-graphics-maintainer@...are.com>,
        <liweihang@...wei.com>, <liangwenpeng@...wei.com>,
        <yishaih@...dia.com>, <zackr@...are.com>, <zyjzyj2000@...il.com>
Subject: Re: [PATCH rdma-next v4 2/3] lib/scatterlist: Fix wrong update of
 orig_nents


On 8/24/2021 10:12 PM, Jason Gunthorpe wrote:
> On Tue, Aug 24, 2021 at 05:25:30PM +0300, Maor Gottlieb wrote:
>> @@ -514,11 +531,13 @@ struct scatterlist *sg_alloc_append_table_from_pages(struct sg_table *sgt,
>>   		offset = 0;
>>   		cur_page = j;
>>   	}
>> -	sgt->nents += added_nents;
>> +	sgt_append->sgt.nents += added_nents;
>> +	sgt_append->sgt.orig_nents = sgt_append->sgt.nents;
>> +	sgt_append->prv = s;
> Why is nents being touched here?
>
> Shouldn't it just be
>
>      sgt_append->sgt.orig_nents += added_nents;
>      sgt_append->prv = s;
>
> ?

In general yes, but all the sg_alloc_* functions update both orig_nents 
and nents (to same value). May be drivers that pass nents instead of 
orig_nents to dma_map_sg* (like umem did before this patch set). So I 
prefer to set it here as well, also not to break 
sg_alloc_table_from_pages_segment which use the same path. This kind of 
change could be done very carefully next cycle for all sg_alloc_*.

>
> Let me know I can fix it
>
> Jason