Messages in this thread Patch in this message | | | Subject | [PATCH] VFS mmap wrong behavior when I/O failure occurs | From | fs <> | Date | Wed, 11 May 2005 14:47:11 -0400 |
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Related FS: EXT2, EXT3, XFS
Related files: fs/buffer.c mm/filemap.c
Bug description: Make a partition in USB storage HDD, create a 64M file. Write a program, do: open - mmap - read memory at offset 16M (from disk) - munmap - close. After each operation, pause for a while, such as 3 seconds. Between mmap and read memory, unplug a USB wire to force a I/O error. The read memory will report no error and get 0x00 back, while the right result is SIGBUS.
Bug analysis: When accessing a page mmaped, kernel calls do_no_page->..->filemap_nopage->mapping->a_ops->readpage to read a page from disk. In filemap_nopage(), if !Uptodate(page) (i.e.: readpage() fails) , it will ClearPageError(page) and try readpage() again. Most FS will call mpage_readpage(). For EXT2/EXT3/XFS, it calls block_read_full_page().
1st readpage()->block_read_full_page(): get_block fails(because of I/O failure), if (iblock < lblock) { if (get_block(inode, iblock, bh, 0)) SetPageError(page); } if (!buffer_mapped(bh)) { void *kaddr = kmap_atomic(page, KM_USER0); memset(kaddr + i * blocksize, 0, blocksize); flush_dcache_page(page); kunmap_atomic(kaddr, KM_USER0); set_buffer_uptodate(bh); //NOTICE HERE continue; } ...
Then ClearPageError(page);
2nd readpage()->block_read_full_page(): for every buffer: if (buffer_uptodate(bh)) continue; So at the end, the page/buffer is uptodate, no Error set. filemap_nopage will happily return a page memset with 0 without any error!
Way around: A. do not set buffer uptodate Or B. do not call readpage() twice.
Patch: Since the buffer is memset to 0, no need to set_buffer_uptodate. diff -uNp linux-2.6.11.8-orig/fs/buffer.c linux-2.6.11.8/fs/buffer.c --- linux-2.6.11.8-orig/fs/buffer.c 2005-05-11 14:41:03.000000000 -0400 +++ linux-2.6.11.8/fs/buffer.c 2005-05-11 14:38:55.000000000 -0400 @@ -2105,7 +2105,6 @@ int block_read_full_page(struct page *pa memset(kaddr + i * blocksize, 0, blocksize); flush_dcache_page(page); kunmap_atomic(kaddr, KM_USER0); - set_buffer_uptodate(bh); continue; } /* Signed-off-by: Qu Fuping <fs@ercist.iscas.ac.cn> ---- Best Regards, Qu Fuping
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