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    SubjectRe: [PATCH] zero_user_page uses in fs/buffer.c and fs/libfs.c
    On Mon, 30 Apr 2007 00:04:31 -0700 (PDT)
    Christoph Lameter <clameter@sgi.com> wrote:

    >
    > Replace open-coded kmap_atomic() and kunmap_atomic()
    > surrounding two memory clear operations with zero_user_page(), as both
    > memory operations act on the same page.
    >
    > Cc: Nate Diller <nate.diller@gmail.com>
    > Signed-off-by: Christoph Lameter <clameter@sgi.com>
    >
    > ---
    > fs/buffer.c | 29 ++++++++++-------------------
    > fs/libfs.c | 10 +++++-----
    > 2 files changed, 15 insertions(+), 24 deletions(-)
    >
    > Index: linux-2.6.21-rc7-mm2/fs/buffer.c
    > ===================================================================
    > --- linux-2.6.21-rc7-mm2.orig/fs/buffer.c 2007-04-27 22:51:27.000000000 -0700
    > +++ linux-2.6.21-rc7-mm2/fs/buffer.c 2007-04-29 23:58:48.000000000 -0700
    > @@ -1796,19 +1796,12 @@ static int __block_prepare_write(struct
    > set_buffer_uptodate(bh);
    > continue;
    > }
    > - if (block_end > to || block_start < from) {
    > - void *kaddr;
    > -
    > - kaddr = kmap_atomic(page, KM_USER0);
    > - if (block_end > to)
    > - memset(kaddr+to, 0,
    > - block_end-to);
    > - if (block_start < from)
    > - memset(kaddr+block_start,
    > - 0, from-block_start);
    > - flush_dcache_page(page);
    > - kunmap_atomic(kaddr, KM_USER0);
    > - }
    > + if (block_end > to)
    > + zero_user_page(page, to,
    > + block_end - to, KM_USER0);
    > + if (block_start < from)
    > + zero_user_page(page, block_start,
    > + from - block_start, KM_USER0);
    > continue;
    > }
    > }
    > @@ -2224,7 +2217,6 @@ int nobh_prepare_write(struct page *page
    > unsigned block_in_page;
    > unsigned block_start;
    > sector_t block_in_file;
    > - char *kaddr;
    > int nr_reads = 0;
    > int i;
    > int ret = 0;
    > @@ -2264,13 +2256,12 @@ int nobh_prepare_write(struct page *page
    > if (PageUptodate(page))
    > continue;
    > if (buffer_new(&map_bh) || !buffer_mapped(&map_bh)) {
    > - kaddr = kmap_atomic(page, KM_USER0);
    > if (block_start < from)
    > - memset(kaddr+block_start, 0, from-block_start);
    > + zero_user_page(page, block_start,
    > + from - block_start, KM_USER0);
    > if (block_end > to)
    > - memset(kaddr + to, 0, block_end - to);
    > - flush_dcache_page(page);
    > - kunmap_atomic(kaddr, KM_USER0);
    > + zero_user_page(page, to,
    > + block_end - to, KM_USER0);
    > continue;
    > }
    > if (buffer_uptodate(&map_bh))
    > Index: linux-2.6.21-rc7-mm2/fs/libfs.c
    > ===================================================================
    > --- linux-2.6.21-rc7-mm2.orig/fs/libfs.c 2007-04-27 22:51:27.000000000 -0700
    > +++ linux-2.6.21-rc7-mm2/fs/libfs.c 2007-04-27 22:55:12.000000000 -0700
    > @@ -338,11 +338,11 @@ int simple_prepare_write(struct file *fi
    > {
    > if (!PageUptodate(page)) {
    > if (to - from != PAGE_CACHE_SIZE) {
    > - void *kaddr = kmap_atomic(page, KM_USER0);
    > - memset(kaddr, 0, from);
    > - memset(kaddr + to, 0, PAGE_CACHE_SIZE - to);
    > - flush_dcache_page(page);
    > - kunmap_atomic(kaddr, KM_USER0);
    > + if (from)
    > + zero_user_page(page, 0, from, KM_USER0);
    > + if (to < PAGE_CACHE_SIZE)
    > + zero_user_page(page, to,
    > + PAGE_CACHE_SIZE - to, KM_USER0);
    > }
    > }
    > return 0;

    As Satyam said, this will sometimes cause us to map and unmap the page
    twice, and to run flush_dcache_page() twice. In not-terribly-uncommon
    circumstances in very frequently called functions.

    Doesn't seem worth it to me.
    -
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