Nick proposed years ago that tmpfs should avoid clearing its pages where
write will overwrite them with new data, as ramfs has long done. But I
messed it up and just got bad data. Tried again recently, it works fine.
Here's time output for writing 4GiB 16 times on this Core i5 laptop:
before: real 0m21.169s user 0m0.028s sys 0m21.057s
real 0m21.382s user 0m0.016s sys 0m21.289s
real 0m21.311s user 0m0.020s sys 0m21.217s
after: real 0m18.273s user 0m0.032s sys 0m18.165s
real 0m18.354s user 0m0.020s sys 0m18.265s
real 0m18.440s user 0m0.032s sys 0m18.337s
ramfs: real 0m16.860s user 0m0.028s sys 0m16.765s
real 0m17.382s user 0m0.040s sys 0m17.273s
real 0m17.133s user 0m0.044s sys 0m17.021s
Yes, I have done perf reports, but they need more explanation than they
deserve: in summary, clear_page vanishes, its cache loading shifts into
copy_user_generic_unrolled; shmem_getpage_gfp goes down, and surprisingly
mark_page_accessed goes way up - I think because they are respectively
where the cache gets to be reloaded after being purged by clear or copy.
Suggested-by: Nick Piggin <npiggin@xxxxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
mm/shmem.c | 20 +++++++++++++++++---
1 file changed, 17 insertions(+), 3 deletions(-)
--- 3045N.orig/mm/shmem.c 2012-05-05 10:46:05.732062006 -0700
+++ 3045N/mm/shmem.c 2012-05-05 10:46:12.316062172 -0700
@@ -1095,9 +1095,14 @@ repeat:
shmem_recalc_inode(inode);
spin_unlock(&info->lock);
- clear_highpage(page);
- flush_dcache_page(page);
- SetPageUptodate(page);
+ /*
+ * Let SGP_WRITE caller clear ends if write does not fill page
+ */
+ if (sgp != SGP_WRITE) {
+ clear_highpage(page);
+ flush_dcache_page(page);
+ SetPageUptodate(page);
+ }
if (sgp == SGP_DIRTY)
set_page_dirty(page);
}
@@ -1307,6 +1312,14 @@ shmem_write_end(struct file *file, struc
if (pos + copied > inode->i_size)
i_size_write(inode, pos + copied);
+ if (!PageUptodate(page)) {
+ if (copied < PAGE_CACHE_SIZE) {
+ unsigned from = pos & (PAGE_CACHE_SIZE - 1);
+ zero_user_segments(page, 0, from,
+ from + copied, PAGE_CACHE_SIZE);
+ }
+ SetPageUptodate(page);
+ }
set_page_dirty(page);
unlock_page(page);
page_cache_release(page);
@@ -1768,6 +1781,7 @@ static int shmem_symlink(struct inode *d
kaddr = kmap_atomic(page);
memcpy(kaddr, symname, len);
kunmap_atomic(kaddr);
+ SetPageUptodate(page);
set_page_dirty(page);
unlock_page(page);
page_cache_release(page);
--
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