Il 25/07/2012 16:36, Boaz Harrosh ha scritto:
>> >
>> > I did test the patch with value-assignment.
>> >
>
> Still you should use the sg_set_page()!!
> 1. It is not allowed to directly manipulate sg entries. One should always
> use the proper accessor. Even if open coding does work and is not a bug
> it should not be used anyway!
> 2. Future code that will support chaining will need to do as I say so why
> change it then, again?
Future code that will support chaining will not copy anything at all.
Also, and more important, note that I am _not_ calling sg_init_table
before the loop, only once in the driver initialization. That's because
memset in sg_init_table is an absolute performance killer, especially if
you have to do it in a critical section; and I'm not making this up, see
blk_rq_map_sg:
* If the driver previously mapped a shorter
* list, we could see a termination bit
* prematurely unless it fully inits the sg
* table on each mapping. We KNOW that there
* must be more entries here or the driver
* would be buggy, so force clear the
* termination bit to avoid doing a full
* sg_init_table() in drivers for each command.
*/
sg->page_link &= ~0x02;
sg = sg_next(sg);
So let's instead fix the API so that I (and blk-merge.c) can touch
memory just once. For example you could add __sg_set_page and
__sg_set_buf, basically the equivalent of
memset(sg, 0, sizeof(*sg));
sg_set_{page,buf}(sg, page, len, offset);
Calling these functions would be fine if you later add a manual call to
sg_mark_end, again the same as blk-merge.c does. See the attached
untested/uncompiled patch.
And value assignment would be the same as a
__sg_set_page(sg, sg_page(page), sg->length, sg->offset);
> Please don't change two things in one patch. The fix is for high-pages
> please fix only that here. You can blasphemy open-code the sg manipulation
> in a separate patch.
The blasphemy is already there (the scatterlist that is handed to virtio
won't have the right end-of-chain marker). If anything,
value-assignment is trading a subtle blasphemy for a blatant one.
That's already an improvement, but let's just fix the API instead.
Paolo
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