On Tuesday May 29, nickpiggin@xxxxxxxxxxxx wrote:
Questions:
- Is this a problem, and should it be fixed (I think "yes").
I think you are right.
That's encouraging - thanks.
- Is the patch appropriate, and does it have no negative
consequences?.
(Obviously some comments should be tidied up to reflect the new
reality).
Would it be better (and closer to following the existing logic) if
we sampled i_size before testing each page for uptodateness? It might
also cost a little less in the fastpath case of finding an uptodate
page.
Uhm. no.
It doesn't address the issue which is that we need to check i_size
*after* we have got hold of the page and believe it to be uptodate.
And just to check, I race the test with your patch and I get a nice
little row of "!!!!!" - it is finding zeros still.
But we could possibly remove the early checks on end_index, that
makes sense (I've also moved the comment).
Thanks,
NeilBrown
Signed-off-by: Neil Brown <neilb@xxxxxxx>
### Diffstat output
./mm/filemap.c | 72 ++++++++++++++++++++++-----------------------------------
1 file changed, 28 insertions(+), 44 deletions(-)
diff .prev/mm/filemap.c ./mm/filemap.c
--- .prev/mm/filemap.c 2007-05-29 16:45:26.000000000 +1000
+++ ./mm/filemap.c 2007-05-29 16:58:23.000000000 +1000
@@ -875,13 +875,11 @@ void do_generic_mapping_read(struct addr
{
struct inode *inode = mapping->host;
unsigned long index;
- unsigned long end_index;
unsigned long offset;
unsigned long last_index;
unsigned long next_index;
unsigned long prev_index;
unsigned int prev_offset;
- loff_t isize;
struct page *cached_page;
int error;
struct file_ra_state ra = *_ra;
@@ -894,27 +892,12 @@ void do_generic_mapping_read(struct addr
last_index = (*ppos + desc->count + PAGE_CACHE_SIZE-1) >> PAGE_CACHE_SHIFT;
offset = *ppos & ~PAGE_CACHE_MASK;
- isize = i_size_read(inode);
- if (!isize)
- goto out;
-
- end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
for (;;) {
struct page *page;
+ unsigned long end_index;
+ loff_t isize;
unsigned long nr, ret;
- /* nr is the maximum number of bytes to copy from this page */
- nr = PAGE_CACHE_SIZE;
- if (index >= end_index) {
- if (index > end_index)
- goto out;
- nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
- if (nr <= offset) {
- goto out;
- }
- }
- nr = nr - offset;
-
cond_resched();
if (index == next_index)
next_index = page_cache_readahead(mapping, &ra, filp,
@@ -929,6 +912,32 @@ find_page:
if (!PageUptodate(page))
goto page_not_up_to_date;
page_ok:
+ /*
+ * i_size must be checked after we know the page is Uptodate.
+ *
+ * Checking i_size after the check allows us to calculate
+ * the correct value for "nr", which means the zero-filled
+ * part of the page is not copied back to userspace (unless
+ * another truncate extends the file - this is desired though).
+ */
+
+ isize = i_size_read(inode);
+ end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
+ if (unlikely(!isize || index > end_index)) {
+ page_cache_release(page);
+ goto out;
+ }
+
+ /* nr is the maximum number of bytes to copy from this page */
+ nr = PAGE_CACHE_SIZE;
+ if (index == end_index) {
+ nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
+ if (nr <= offset) {
+ page_cache_release(page);
+ goto out;
+ }
+ }
+ nr = nr - offset;
/* If users can be writing to this page using arbitrary
* virtual addresses, take care about potential aliasing
@@ -1015,31 +1024,6 @@ readpage:
unlock_page(page);
}
- /*
- * i_size must be checked after we have done ->readpage.
- *
- * Checking i_size after the readpage allows us to calculate
- * the correct value for "nr", which means the zero-filled
- * part of the page is not copied back to userspace (unless
- * another truncate extends the file - this is desired though).
- */
- isize = i_size_read(inode);
- end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
- if (unlikely(!isize || index > end_index)) {
- page_cache_release(page);
- goto out;
- }
-
- /* nr is the maximum number of bytes to copy from this page */
- nr = PAGE_CACHE_SIZE;
- if (index == end_index) {
- nr = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
- if (nr <= offset) {
- page_cache_release(page);
- goto out;
- }
- }
- nr = nr - offset;
goto page_ok;
readpage_error: