On Wed, 2013-01-16 at 12:07 +0530, Subhash Jadavani wrote: > On 1/15/2013 9:49 PM, James Bottomley wrote: > > On Tue, 2013-01-15 at 21:31 +0530, Subhash Jadavani wrote: > >> blk_rq_map_sg() function merges the physically contiguous pages to use same > >> scatter-gather node without checking if their page descriptors are > >> contiguous or not. > >> > >> Now when dma_map_sg() is called on the scatter gather list, it would > >> take the base page pointer from each node (one by one) and iterates > >> through all of the pages in same sg node by keep incrementing the base > >> page pointer with the assumption that physically contiguous pages will > >> have their page descriptor address contiguous which may not be true > >> if SPARSEMEM config is enabled. So here we may end referring to invalid > >> page descriptor. > >> > >> Following table shows the example of physically contiguous pages but > >> their page descriptor addresses non-contiguous. > >> ------------------------------------------- > >> | Page Descriptor | Physical Address | > >> ------------------------------------------ > >> | 0xc1e43fdc | 0xdffff000 | > >> | 0xc2052000 | 0xe0000000 | > >> ------------------------------------------- > >> > >> With this patch, relevant blk-merge functions will also check if the > >> physically contiguous pages are having page descriptors address contiguous > >> or not? If not then, these pages are separated to be in different > >> scatter-gather nodes. > > How does this manifest as a bug? > > > > The discontinuity is in struct page arrays, which hardware doesn't care > > about. All we need is to get from struct page to the physical address > > for programming the hardware, for which we use the sg_phys() inline > > function. > > > > Even given we have a two page physical contiguity at 0xdffff000 in your > > example, the sg list entry contains a length of 8192 and a page_link of > > 0xc1e43fdc, which we transform to the correct physical address and > > length. > Thanks James for looking at this patch. > > Let's assume this scenario. > PAGE_SIZE = 4096 (4K), > 2 page descriptors (as mentioned commit text) whose own addresses are > discontingous but they point to physically contiguous memory space, > sizeof(struct page) is 36 bytes. > Only one SG node created in Scatter Gather list (by blk_rq_map_sg) with > this node's page_link=0xc1e43fdc, length=8192, offset=0 > > Now consider this call stack from MMC block driver (this is on the ARmv7 > based board): > [ 98.918174] [<c001b50c>] (v7_dma_inv_range+0x30/0x48) from > [<c0017b8c>] (dma_cache_maint_page+0x1c4/0x24c) > [ 98.927819] [<c0017b8c>] (dma_cache_maint_page+0x1c4/0x24c) from > [<c0017c28>] (___dma_page_cpu_to_dev+0x14/0x1c) > [ 98.937982] [<c0017c28>] (___dma_page_cpu_to_dev+0x14/0x1c) from > [<c0017ff8>] (dma_map_sg+0x3c/0x114) > [ 98.947169] [<c0017ff8>] (dma_map_sg+0x3c/0x114) from > [<c0532c40>] (msmsdcc_prep_xfer+0x50/0x10c) > [ 98.956020] [<c0532c40>] (msmsdcc_prep_xfer+0x50/0x10c) from > [<c0539664>] (msmsdcc_pre_req+0x78/0x98) > [ 98.965237] [<c0539664>] (msmsdcc_pre_req+0x78/0x98) from > [<c0521d98>] (mmc_start_req+0x4c/0x1c4) > [ 98.974088] [<c0521d98>] (mmc_start_req+0x4c/0x1c4) from > [<c052faa0>] (mmc_blk_issue_rw_rq+0x3a0/0x84c) > [ 98.983457] [<c052faa0>] (mmc_blk_issue_rw_rq+0x3a0/0x84c) from > [<c05304b4>] (mmc_blk_issue_rq+0x568/0x5c4) > [ 98.993193] [<c05304b4>] (mmc_blk_issue_rq+0x568/0x5c4) from > [<c0530718>] (mmc_queue_thread+0xb4/0x120) > [ 99.002563] [<c0530718>] (mmc_queue_thread+0xb4/0x120) from > [<c0096400>] (kthread+0x80/0x8c) > [ 99.010987] [<c0096400>] (kthread+0x80/0x8c) from [<c000f028>] > (kernel_thread_exit+0x0/0x8) > > dma_cache_maint_page() function iterates through each page in single sg > node, maps it to virtual space and then call the cache maintainance > operation on that page. > > With above scenario, first page descriptor would be 0xc1e43fdc, which > would be mapped virtual address by either kmap() (if it's higmem page) > or by page_address() (if it's low mem page). > > Now as the size of the sg node is 8192 bytes, dma_cache_maint_page() > function increments the page pointer (pointed by page_link of sg node) > to get to the next descriptor. page++ would yield page descriptor > address = 0xc1e44000 (0xc1e43fdc + sizeof (struct page)). 0xc1e44000 is > not pointing any real page descriptor so when calls > page_address(0xc1e44000) to get virtual address of the page, it returns > invalid virtual address. Now when we try to dereference that invalid > virtual address in cache maintainance functions, it would result into > "Unable to handle kernel paging request at virtual address xxxxxxxx" > error and kernel crashes. Well, that means the fault is in the iterator, doesn't it? It's assuming two pages which are physically contiguous have adjacent entries in the page arrays. If you want to fix that fault, fix it in the iterators ... probably they should use the correct page to pfn transforms which will move across the discontiguous arrays. More simply, though, if the MMC drivers want to iterate through all the pages in the array, then surely you don't want any physical merging anyway, so your quick fix is to turn off clustering: q->limits.cluster = 0; in the mmc layer? James -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
