Subject: user memory access problem

From: Andre Schwarz <[email-masked]>
Date: Tue, 17 Jun 2008 14:41:55 +0200

Actually we're running 2.6.26-rc6 with MPC8343 and MPC5200B based boards.

Our kernel driver implements a read functions that initiates a DMA
operation into that user memory - nothing special.
For testing purposes this memory can be touched from the kernel using
memset.

1. User memory is translated to sg by get_user_pages during read syscall.
2. walk sg entries and map each page using kmap
3. memset the kmap'ed mem
4. SetPageDirty and unmap.

Running this code leads to memory corruption and SegFaults on the
Application side.
This code worked fine on 2.6.19.

Can anybody help me out here ?

"um" is a pointer to an internal struct holding the sg_list besides
other information...

                for (i=0; i<um->page_cnt; i++) {
                        data = i & 0xff;
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,25)
                        kva = kmap(um->sl[i].page) + um->sl[i].offset;
#else
                        kva = kmap(sg_page(&um->sl[i])) + um->sl[i].offset;
#endif
                        if (kva) {
                                /*    leads to SegFault ...   */
                                memset (kva, data, um->sl[i].length);
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,25)
                                SetPageDirty(um->sl[i].page);
                                kunmap(sg_page(um->sl[i].page));
#else
                                SetPageDirty(sg_page(&um->sl[i]));
                                kunmap(sg_page(&um->sl[i]));
#endif
                        }
                }

regards,
Andre Schwarz
Matrix Vision

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