Messages in this thread | | | Date | Mon, 22 Nov 2004 02:41:45 -0800 | From | Andrew Morton <> | Subject | Re: [RFC][PATCH] problem of cont_prepare_write() |
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OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> wrote: > > If I do the following operation on fatfs, and my box under heavy load, > > open("testfile", O_CREAT | O_TRUNC | O_RDWR, 0664); > lseek(fd, 500*1024*1024 - 1, SEEK_SET); > write(fd, "\0", 1); > > In cont_prepare_write(), kernel fills the hole by zero cleared page. > > fs/buffer.c:cont_prepare_write:2210, > while(page->index > (pgpos = *bytes>>PAGE_CACHE_SHIFT)) { > > [...] > > status = __block_prepare_write(inode, new_page, zerofrom, > PAGE_CACHE_SIZE, get_block); > if (status) > goto out_unmap; > kaddr = kmap_atomic(new_page, KM_USER0); > memset(kaddr+zerofrom, 0, PAGE_CACHE_SIZE-zerofrom); > flush_dcache_page(new_page); > kunmap_atomic(kaddr, KM_USER0); > __block_commit_write(inode, new_page, > zerofrom, PAGE_CACHE_SIZE); > unlock_page(new_page); > page_cache_release(new_page); > } > > But until ->commit_write(), kernel doesn't update the ->i_size. Then, > if kernel writes out that hole page before updates of ->i_size, dirty > flag of buffer_head is cleared in __block_write_full_page(). So hole > page was not writed to disk.
But the page remains locked across both the ->prepare_write() and ->commit_write() operations. So writeback cannot get in there to call ->writepage().
The page lock should correctly synchronise the prepare_write/commit_write and writeback functions.
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