Nick Piggin <npiggin@xxxxxxx> wrote:
>
> Hi Linus,
>
> What do you think about the following couple of patches? Hugh's had a
> look and thinks they're OK.
>
Gad. You're brave.
> Allocate a compound page for the user mapping instead of tweaking
> the page refcounts.
>
> Signed-off-by: Nick Piggin <npiggin@xxxxxxx>
>
> Index: linux-2.6/drivers/scsi/sg.c
> ===================================================================
> --- linux-2.6.orig/drivers/scsi/sg.c
> +++ linux-2.6/drivers/scsi/sg.c
> @@ -1140,32 +1140,6 @@ sg_fasync(int fd, struct file *filp, int
> return (retval < 0) ? retval : 0;
> }
>
> -/* When startFinish==1 increments page counts for pages other than the
> - first of scatter gather elements obtained from alloc_pages().
> - When startFinish==0 decrements ... */
> -static void
> -sg_rb_correct4mmap(Sg_scatter_hold * rsv_schp, int startFinish)
> -{
> - struct scatterlist *sg = rsv_schp->buffer;
> - struct page *page;
> - int k, m;
> -
> - SCSI_LOG_TIMEOUT(3, printk("sg_rb_correct4mmap: startFinish=%d, scatg=%d\n",
> - startFinish, rsv_schp->k_use_sg));
> - /* N.B. correction _not_ applied to base page of each allocation */
> - for (k = 0; k < rsv_schp->k_use_sg; ++k, ++sg) {
> - for (m = PAGE_SIZE; m < sg->length; m += PAGE_SIZE) {
> - page = sg->page;
> - if (startFinish)
> - get_page(page);
> - else {
> - if (page_count(page) > 0)
> - __put_page(page);
> - }
> - }
> - }
> -}
What on earth is the above trying to do? The inner loop is a rather
complex way of doing atomic_add(&page->count, sg->length/PAGE_SIZE). One
suspects there's a missing "[m]" in there.
Yes, using a compound page for the refcounting sounds sane, but I think
this code is fragile and has monsters in it.
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