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    SubjectRe: [PATCH 03/11] vfs: Add better VFS support for page_mkwrite when blocksize < pagesize
    On Thu, May 28, 2009 at 09:03:41AM -0400, Josef Bacik wrote:
    > On Wed, May 27, 2009 at 03:01:00PM +0200, Jan Kara wrote:
    > >
    >
    > Sorry, another possibly braindead question. When we extend the i_size you make
    > it so that the page will be faulted the next time it's written to via
    > page_mkclean, which from what I can tell is done via pte_wrprotect. The problem
    > with this is the next time we write to the page, we do pte_mkwrite, which makes
    > it so that we won't fault the next time we write, correct? So if I were to do
    >
    > ftruncate(fd, 0);
    > pwrite(fd, buf, 1024, 0);
    > map = mmap(NULL, 4096, PROT_WRITE, MAP_SHARED, fd, 0);
    > map[0] = 'a';
    > ftruncate(fd, 10000);
    > map[0] = 'b'; --> causes a page fault again


    This will do block allocation for the entire page within i_size. (ie multiple blocks)
    ext4_page_mkwrite have

    size = i_size_read(..)
    if (page->index == size >> PAGE_CACHE_SHIFT)
    len = size & ~PAGE_CACHE_MASK;
    else
    len = PAGE_CACHE_SIZE;


    > memset(map, 'a', 4096); --> wouldn't cause a pagefault
    > writepage at some point
    >
    > We'd still be in the position that you are trying to solve, correct, since we
    > will have dirtied the rest of the page without calling mkwrite for the other
    > sections of it, which would result in unallocated blocks when we hit writepage.
    > Thanks,
    >

    -aneesh


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